Sure. malloc'ed item could be the same as malloc_gc'd, but with its reference count artificially bumped up by 1. free() would decrement the count and the item

Sure. malloc'ed item could be the same as malloc_gc'd, but with its reference count artificially bumped up by 1. free() would decrement the count and the item will be picked up by the GC later on. If there's a provision for an immediate disposal of unreferenced items, then we are back to the standard malloc/free behaviour in case when malloc and malloc_gc items are not mixed.