In case anyone else has made one of these, I have recently made some software changes. The biggest new feature is that calibration is supported directly from the power supply - no connection to a computer needed. The reason for this is that I have had nothing but problems with the RawHID Python support on Linux and OSX; setting it up was nearly impossible, and if you did manage to get it installed (with custom compiled code) it would just segfault when you try writing stuff. After many hours of debugging it, I gave up.
In addition to adding calibration, I did some code cleanup, updated the code to use my latest build scripts and toolchain, and fixed some minor bugs that I had never bothered fixing before.
The latest code is on my github page: https://github.com/thebiguno/microcontroller-projects/tree/master/projects/power_supply/avr