Sorry for coming into this late, but could this possibly be resolved (in this particular case, and maybe in others) by having the gc preferentially releasing handles that are members of objects with no destructors?
That is, in this case the gc would first release the Character::avatar handle. At this point, the Avatar would have no more references, and could be destructed safely.
Of course, this solution doesn't quite work if you have circular references between classes that each have destructors, but then that is the programmer's own fault, right?
This method of breaking circular references is used by Python and the Boehm GC, although the way each of these handle the case of circular references among objects that all have destructors is to never collect the objects . . . I think that your solution is better in this case. Or, perhaps, throw an exception and refuse to run the destructors. That sounds more complicated, though.