It looks like the runtime _does_ give you this capability (as Adam explains in
his reponse), so I stand corrected on that point. But a compile time solution
is still impossible due to D's compilation model.

A possible, partial solution is to get a module members at compile-time and iterate on them to see if they are subclasses of A. That's partial for the reasons you cited, but that might be enough for the OP.

I don't remember: is there a way to get all visible symbols in the current scope?