So for completeness's sake, I logged into the root account and started X from it. glxinfo shows it using Gallium on an R350, so it looks like it definitely is an issue with permissions. I guess I'll try chmodding those devices and see if that works.
Update: works perfectly, and now compton works with WindowMaker, which was what I was hoping for. Thanks for the help!
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Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
Synthesizers: Roland JX-10/MT-32/D-10, Oberheim Matrix-6, Yamaha DX7/FB-01, Korg MS-20 Mini, Ensoniq Mirage/SQ-80, Sequential Circuits Prophet-600, Hohner String Performer
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