I would think it should be possible, but the whole sparc/sparc64 divide is something I don't really quite understand. I'm not sure what a "sparc64 porterbox" would look like. Are you asking for an environment where all of the userland is 64-bit binaries? If so, I don't think debian has a true sparc64 porterbox yet -- mostly chroots from what I understand. But from a technical standpoint, there are a number of reasons why pure 64-bit environments aren't desired, and why there hasn't been much of a rush to get them.
So all of that having been said, are you trying to create a 64-bit gcc binary in "pure" 64-bit environment? Or just a gcc that can build 64-bit sparc programs on sparc?