Starting from the standard config file in /boot/ and applying the RT patch (and related features in menuconfig) should be really safe on ppc too. Once you get a bootable kernel you can prune it step by step according to your hardware.
I always suggest this howto (apolgize for repetitiveness) except for alsa compilation (we are on debian ;-) ) and for OSS support which I always turn off during menuconfig.
Furthermore, regarding the make-kpkg command is better to issue something like:
make-kpkg --append-to-version -rt-xx --revision xx --initrd kernel_image
Raffaele, thanks for the info. I'm not sure how well i'd do making an RT kernel for a powerpc build. I couldn't get it working when i tried Ubuntu, but i just might have another try with Debian ppc, and see what happens.