Would it be possible to emulate toolboxes on one end for the 68k app and then on the other end provide Mac OS X toolbox hooks that reflect and affect the state of the emulated toolboxes? The overhead seems pretty big, but maybe with a fast Mac it would be tolerable?

Are the toolboxes so massive that any attempt to do this is not humanly possible? We could start with System 6 if that would help.

Or is there a simpler, manual approach - is there a way to break an app back down to its "source code", tweak it yourself to work again, and recompile to a different platform with human intervention?