No this won't accomplish the example I gave in the original post. I need to:

1) Start with C main (so, it needs to be a perl library, not a modified perl)
2) Call perl from C
3) From inside the perl subroutine, call a C function in the same namespace as the C main()

This is what I'm looking for. I'm beginning to think it can't currently be done because it would be too difficult to manage the stack, and probably nobody has needed it since most are happy with Inline, XS or perl_call.