I just discovered that the instructions that turn up for opening a Mac Mini G4 are unnecessarily brutal and prone to gouging or cracking plastic. Instead of prying anything, here's the technique I intuited and used:

1. Use one hand to apply pressure between the back I/O shield and the outer frame (like tensioning the cylinder, if you watch lockpicking videos)

2. With a THIN metal spudger/pry tool, work your way around the edge starting at the back, pressing each clip just enough to release it.