Status update:
Lacking actual hardware I tried to re-bootstrap GPRbuild by doing Koji builds of selected package versions in a specific order. That failed when it turned out that GPRbuild failed to recognize GCC as a suitable compiler.
After lots of trouble and setbacks I finally managed to get a ppc64 emulator installed and working just about well enough that I could build with RPMbuild and troubleshoot GPRbuild. I now have GPRbuild 2015 built for ppc64, and it seems to be able to rebuild itself.
I had no success with any other emulators, but now I have a good idea of how to get GPRbuild to find the compiler on other architectures, so I'm going to try bootstrapping through Koji again.
If I don't run into any more problems, then up-to-date packages may start to appear soonish.
Björn Persson