First, you need to ask your instructors and senior students about anything you see them turning their noses up to. Do this reflexively. Always ask why.
If they have no good answer for you, other than that they just don't do it, then that's your answer. Otherwise, this is the sort of question that will lead to a much better understanding of Brazilian Jiujitsu, because it could reveal something to you that you hadn't considered before.
Wrist locks and wrist take-downs are allowed in most BJJ competitions, with restrictions on children and for some belt ranks (typically not allowed until purple belt).
Wrist locks are in the official Gracie Jiujitsu syllabus, also. They appear in its classical self-defense drill sets. And there are some BJJ black belts who specialize in it and even put on workshops detailing wrist locks in BJJ.
If wrist techniques are legal to do and part of the art, then why would some BJJ people be uncomfortable with them?
One answer is that some people have never seen them, because they either do a sport version of BJJ, or they just haven't reached the rank where wrist locks are introduced. Naturally, they're not going to know what to do right away when you do your wrist lock on them. It might take your instructor stopping class to demo what to do in this case. Then after that, you might find your wrist lock becomes ineffective on others in your class.
In sport BJJ, wrist techniques are considered "low percentage" and may be excluded from practice altogether from a particular school. In other words, they're risky to do and put you in jeopardy of losing your positional advantage or getting submitted.
That being said, even if you do a self-defense based version of BJJ, you might find that your instructors are reluctant to use wrist manipulation. There are good reasons for that.
In the case of the Hapkido "outside wrist throw" that you mentioned (which I think is the same as the Aikido "kote-gaeshi" technique more or less), there are many ways this can go wrong for you. First, realize that someone who is fighting you isn't going to give you their wrists. In fact, the hands are typically the fastest moving body part on your opponent. They flail around so fast your eyes can't even see them. So if you're actively targeting them, trying to capture them and then do your wrist lock throw, you're likely going to fail miserably. You'll miss.
Instead, one of the principles of BJJ is to concentrate on the body first during a take-down, not on the extremities. The body (torso) is the slowest part of your opponent. That's why BJJ is so successful at diving under punches and doing a two-arm grab of the torso before the take-down. Even if you end up eating that punch, you still find yourself holding on to that torso, which allows you to take your opponent down and win the fight on the ground.
Second, the way this wrist throw is done, you typically have to get some distance between you and your opponent, and turn perpendicular to your opponent or turn your back against your opponent while applying force to the hand and wrist. This is problematic for you.
Getting distance implies you were close to your opponent already and then got out of there. There are easier ways of taking him down if you managed to get close. You just lost that advantage if you broke away. And you took extra time to get that distance, during which time your opponent can follow you and take you down. Also, at this distance, you're vulnerable to being struck with punches and kicks (and your opponent has a free arm and two free legs). BJJ likes to quickly get in and control the torso, because in that short distance, your opponent can't generate as much power in his strikes.
If you never got close in the first place prior to performing the wrist lock, then see my first point, above: People don't just let you grab their flailing wrists, unless you get lucky, but luck isn't something you should train to rely on.
Also, turning your body perpendicular to your opponent like this technique does means you are opening yourself up to being taken down. It's not a solid, defensive stance.
And using both of your hands to apply the wrist lock throw means both of your arms are busy doing something, while only one of his arms is captured. If you're doing the math, that means he has one more arm than you do free. Those are bad odds. And as I mentioned before, he still has the ability to kick and punch you, whereas your arms are tied up trying to do a wrist throw, unable to guard and block.
There are many Hapkido and Aikido people that just cringed upon reading that, I'll bet. They're recalling all of the times it worked for them in class, (typically against lapel grabs from stiff-armed people who are told to keep holding onto that lapel).
The key thing to realize is that these arts don't typically involve actively resisting opponents. When that element (sparring) is added, these wrist throws are almost never done. People don't let you take their wrists without punching or kicking the crap out of you, basically.
All that being said, do a Youtube search for "BJJ wrist locks". You might be surprised by what you see. There are a lot of videos showing them. But pay attention to the fact that none of them look anything like the outside wrist throw like you mentioned. Instead, they tend to be done from very close in when it's safe for them to do so.
BJJ people can be very sneaky about their wrist locks. You might not see them unless someone pointed it out. It's not that they're illegal moves, so people are trying to disguise them. No, they're all perfectly legal moves in BJJ. It's just that the wrist lock is generally not the only thing they have going on in the technique. It's like a cherry on top or the icing on the cake. It can and has been used to tap opponents, but it's most often used transitionally, to move into a better position.
It's probably something you should work on at black belt level. Until then, you might want to stick with the things your system is trying to teach you.
As for other techniques which come from other martial arts styles, besides wrist locks, this depends on the school and the instructor. People are there to learn BJJ, and so if you come into the school with stuff from other martial arts, it might be seen as, "That's nice, now how about you concentrate on learning what you're here to learn?" (But definitely follow-up on it with your instructor asking him why those techniques are frowned upon.)
The BJJ schools I've been to have all seemed to welcome other outside martial arts techniques. For example, one school I visited had a high ranking Judo student. So the 3rd degree black belt instructor asked the Judo student to show the class the proper way to throw. And apparently this was a regular part of their class, to ask her for correction on their throws.
I've seen other BJJ schools that have ex-wrestlers, muay-thai people, boxers, and kali people who demonstrate their stuff for the class. In general, it's a small part of the class, but the instructors welcome it.
The etiquette is very important, though. You can't just yell, "Hey everyone, look at me. I'm going to show you this cool Hapkido technique." Instead, it has to be something the instructor has seen you do before, asked you about, and had a chance to test you with it. Then in class, if he sees merit to the technique and thinks you're particularly good at it, he might turn to you and ask you to demonstrate it. Otherwise, it's best to keep it to yourself in class and work on it only when you're rolling outside of class time.
My perspective anyway. For what it's worth.
Hope that helps.