Yes, however then for stability you need to sugar *all* Scheme object allocations you do in C with heap overflow handling logics.

Also a route would be you made a c-define that you have a C enumeration loop that reports to and the invoked Scheme code does the allocation. For stability you'd need to ensure other threads don't take over execution and rewind another C frame, and that you do by stopping Gambit's scheduler, and also if there'd be a heap overflow or other exception, that needs to be handled in such a way that your C enumeration loop's stack frame is rewound properly, so unless needed you don't want to take this route.

So I'd say as long as you feel it's viable to go with the form you are with now, that is the best option.