> Any malloc'd pointer will look just like a GC pointer, so you'll be paying the penalty for all of those.

I don't see why. A small amount of manual tagging for extern(C) things—done in this case as part of the standard library, so opaque to user code—will go a long way in that regard. And in any custom allocator, automatic inference should work just fine, since the data returned will be clearly derived from the already-tagged malloc/mmap/VirtuallAlloc/whatever (type info does still have to be associated manually, of course, as part of the GC.addRange dance, to avoid spurious pinning).