I agree with this, but I suspect that there might be a better solution in between.
Freescale Semiconductor produces Power architecture development boards that could be sourced and replaced without having to rely on second hand hardware, and they have versions that run on ARM too. They're designed to be stacked with custom made LCD screens and they come with a Linux distro. I see no reason why you can't upload a custom OS to one of these to play around with before scaling it down to work with older Macs and the "old world"/"new world" firmware.
(Bearing in mind that I do very little OS development myself, so there may be significant roadblocks that I'm overlooking. If NetBSD can be made to run on a Mac Quadra, though, this doesn't seem like so much of a leap.)
EDIT: And of course, Freescale still sells a version of CodeWarrior for these PowerPC boards. It's about as close as you can get to PPC Mac development with today's hardware and software.
Freescale Semiconductor produces Power architecture development boards that could be sourced and replaced without having to rely on second hand hardware, and they have versions that run on ARM too. They're designed to be stacked with custom made LCD screens and they come with a Linux distro. I see no reason why you can't upload a custom OS to one of these to play around with before scaling it down to work with older Macs and the "old world"/"new world" firmware.
(Bearing in mind that I do very little OS development myself, so there may be significant roadblocks that I'm overlooking. If NetBSD can be made to run on a Mac Quadra, though, this doesn't seem like so much of a leap.)
EDIT: And of course, Freescale still sells a version of CodeWarrior for these PowerPC boards. It's about as close as you can get to PPC Mac development with today's hardware and software.