I think thats quite a difficult thing to do unless you dont mind having some really wierd shaped asteroids. Maybe reach out to @Colludium who might share his physics object connecting secrets
Thanks for the vote of confidence!
@CrudeMik, I'll help if I can - I will ask some questions as we go... The most important question is do you really need to use physics for this? The native box2d web, asm.js and also r0j0's chipmunk physics are all very good at letting you create simple joined shapes with a few objects linked using distance joints or limited revolute joints. But, as soon as you exceed about 10 connected objects then you almost invariably end up in a land of joint fighting and exploding object groups!
IMO, up to 50 objects might be possible using C2 in zero gravity, but there would be serious limitations to how that group could interact with other objects in the layout; the fewer the better for consistent behavior. As soon as something bumps into a large group like that then the you run the risk of everything falling apart (a problem that is exacerbated on older hardware - it's worth seriously considering allowing the behavior to appear to slow down with an fps drop rather than let it compensate - fixed time steps rather than framerate independent could be the way to go).
All of this is probably not just a limitation of c2 and probably applies to most other 2d game physics engines. My Dynamic Sketch Book game joins objects of low mass and low restitution by using lots of distance joints - I found that limited revolute joints were too prone to fighting, and I also found that external interactions with the new group were not good (prone to cause the group to explode). Zero gravity is less stressful on a large group, so you might get away with it if you're careful.
It's also worth considering if you could get the same effect by making all of the asteroids immovable physics objects and just have everything else in the layout move around them... Then you could have amazingly complex shapes with no interaction problems!