Port NextSTEP/OpenStep/GNUstep?
http://en.wikipedia.org/wiki/Nextstep
http://en.wikipedia.org/wiki/GNUstep
http://en.wikipedia.org/wiki/OpenStep
http://www.nextarchive.net/
Ok, so not exactly "classic Mac app compatible", but with Objective C onboard, you could code apps that can be compiled to run on either 68k machines or under OS X