I still don't understand if multiarch will solve the main problem: does it prevent configure scripts to consider different multiarch as cross-compilation? Probably, yes. But I guess that configure scripts must be multiarch-aware.
Yes, multiarch must be supported by given platform, the Debian wiki describes a couple of platforms (MIPS, PowerPC, Intel, ...) but m68k is not among them :-( Who knows, maybe it is not that hard.
Btw, I tried to build a multiarch gcc (x64) on my 32-bit Arch Linux, it seems to do its job well but since Arch doesn't support this, build stopped right on the first -m64 switch, complaining about lack of x64 libraries (what is true).