Garbage collector

I was wondering if it is indeed possible to implement a garbage
collector in Eiffel itself.
AFAIK "disposing" could be implemented using extensively internals;
creation is still impossible to hack at Eiffel level AFAIK.
Beside the obvious support required by the compiler to customize object
creation from Eiffel, is there any specific reason to keep the GC
implemented in C?

Now, to add some spice to the discussion, a thread safe runtime could
allow the GC to run as an underground thread, couldn't it... well,
whatever thread safe means 8-P

/me this evening will dig programming books on thread safety.