Well, the idea is good and it did cross my mind.  Particularly it could be useful for performance sensitive applications.
However it complicates things.
1) If a GC is rejected, when do we make the next attempt?
2) If a callback cancels GC, then what about the other callbacks that have already been called?  They would need to get some "canceled" call.  But this is ok, I suppose.

Since we have already passed the beta 2, I'll see if I can come up with a simple change, particularly if we don't descend into some infinite loop wrt. 1) above.