There is a project to use the Snow Leopard kernel and open source components and some bits from the prerelease Snow Leopard on PPC to create a working PPC Snow Leopard.

I wonder if something like that might be a short term viable path to get working apps. Something like the non-opensource libraries as a download pack for certain each release of Mac OS X, for at least versions Apple no longer supports, to get working programs, then you could slowly work toward open implementations.