I think this comes down to a question about what you really want. Do you want to really understand grappling, or do you want to be shown how to see the grappling within what you've already learned from kung-fu?
If you want to understand grappling, do a style that focuses on grappling. Those styles include, but are not limited to: Brazilian Jiujitsu, Judo, Sambo, Wrestling, classical Japanese jujitsu, Bujinkan, and Shuai-jiao.
Eagle claw, hsing-yi, northern mantis, etc. are not going to give you a well rounded understanding in grappling. These styles all have joint manipulation and throws in them, but it's going to be pretty limited in scope. More importantly, it's how they train that really matters.
It's simple to see for yourself. If you go to one of those kung-fu schools and observe a class or two, you're going to see that they devote just a small percentage of their time to grappling. And when they do any sort of grappling, chances are it will be with compliant partners all the time. Compliant partners do not give you the skill needed to apply grappling techniques under pressure, such as in a real fight. Observe also the types of grappling techniques being taught. Are the techniques against a broad range of realistic attacks? Or do they seem to concentrate on standing arm-bars and wrist locks? Do you see any realistic take-down defense? Do you see any realistic ground fighting?
There are chin-na and throws in just about every form of kung-fu. Bagua is a very throw-oriented form of kung-fu, for example. Taiji is filled with throws and chin-na. Hsing-Yi has it, too. Southern Mantis has a lot of standing arm-bars, arm-breaks, and sweep throws. But all of these are going to be pretty superficial and incomplete, even with an instructor who can show you it. It's not going to be pressure tested, either.
You may be able to find an instructor who can show you where it's at in all the forms you've learned and have fun that way. It doesn't even have to be an instructor within your style, because grappling is fairly universal. It sounds like that may be what you really want.
That is a very enlightening and cerebral thing to do. I should know, because I went through that phase myself. But I personally learned to see it in my kung-fu forms only after learning classical Japanese Jujitsu. After that, things became pretty obvious, because there's only so many ways the body can move. And function dictates form.
But there's still the matter of being able to use it under stress, if that matters to you. That depends on how you train. Again, if you have compliant partners who stop and let you do stuff to them, that's fairly useless to you for self-defense. If, on the other hand, your training partners actively resist everything you do, then you have a chance. Figuring that out on your own is hard, though. You're better off doing BJJ, Judo, Sambo, Wrestling, or Shuai-Jiao, because they already practice this way. (And FYI, classical Japanese jujitsu and Bujinkan do not train this way, at least not very often.)
Hope that helps.