Stack copying strikes me as oh so wrong... Anyways, allocating frames or stacks on the heap doesn't seem like it should require teaching the GC all that much.

Stack copying strikes me as oh so wrong... Anyways, allocating frames or stacks on the heap doesn't seem like it should require teaching the GC all that much. They would essentially be objects of such a class that the GC can traverse them... just like a normal stack (from current frame through return closures), which it already has to know how to do. Their location shouldn't be so important.