Not sure if there has been any effort on such, but this would probably allow all modern code with an OS X port to be back ported to OpenSTEP... My keen goal would be Firefox at a minimum. Need an SSL capable browser. Not sure if it'd need pieces of something like Cocotron or Gnustep. Ideally it would be a modular plug-in upgrade for the dev tools on 4.2 natively but a cross-compiler for OS X might work OK and be much easier.
Long way down the road would be to add 68K back in... not sure about 3.x
Thoughts?
xCode is actually based on the OSX Developer tools, which are in turn based on the Next/Openstep developer tools.