That sounds like a great question for your doctor! Honestly, that's who you should be talking to about this. Your doctor can do a bone density scan and look over your medical history to know exactly what you can and can not do.
Incidentally, I'm not a doctor, but I do know that bones heal themselves over time. Fractures, cracks, and weak areas can get better over time.
Bone is not dead tissue. It's actually alive, filled with cells that are capable of growing more bone in response to various conditions.
One of those conditions that grows bone is weightlifting. Any kind of strength training exercise or resistance-based exercise involving the arm would help. Pull-ups, push-ups, dead-lifts, and presses.
I don't think your bone injury necessarily means you can't train in BJJ, but your instructor and training partners will need to know about it so that they don't apply too much torque to it.
It would help your partners if you wore a full-arm rash guard on that arm only (the other arm's rash guard you can cut off with scissors) to let them "feel" when they have the arm that shouldn't be torqued. When doing gi work, have something on your gi's sleeve that they can feel (like PlastiDip) to identify the arm they need to watch out for.
As for specific techniques within BJJ that cause force on the arm bone itself, there aren't any that I can think of at the moment. Submissions work on weak points on the body, and bones are typically the strongest points. Looking at juji-gatame (arm bar) for example, the force is directed at the elbow joint, not at the bones of the arm itself.
EDIT: Actually, as someone else pointed out, there's the Bicep Slicer technique. It's a technique whereby you're putting pressure on the bicep muscle. That force drives the muscle into the humerus (upper arm) bone. And since leverage is used, it can snap the bone, especially if your bone is weak. It isn't actually meant to attack the bone, just the bicep muscle. But in your case, your bone might be vulnerable. This technique is more advanced, and probably you'll be ready for it when you encounter it later on in your training. The main thing to remember is: Tap soon, and tap often.
Where you might encounter problems is when you're trying to pull your arm out of submissions such as the arm-bar. That would put force on the bone itself. Who knows, maybe with enough force, it could snap.
Another thing you'll encounter often is putting your hands down on the mat to protect yourself from a throw or a take-down. If you're not careful, you can lock your arms out and cause a weak bone to fracture. But usually stiff arms during a breakfall causes shoulder and wrist injuries. The problem is that it's very easy to forget how to fall correctly, and your instincts take over. That's when you get into trouble.
The Oma-Plata technique can be trouble for you, also. It doesn't necessarily target the bones, but people often put their knees down on your arm and put their full body weight on it. This happens to the humerus (upper arm) bone. The ulna and radial bones of the lower arm can be torqued also with this technique, as with the kimura technique.
So this is why you have to talk with your partners ahead of time. And go slow and light at first, until you can sense what's going on and can defend your arm better.
Your doctor will have the ultimate say, though.
Hope that helps.