I guess I confused the issue by referring it as the "radeon driver", but I meant to implicate the radeon.ko kernel module, not the radeon(4) Xorg driver. The servers I'm talking about don't even have X installed (they're Ceph OSD nodes).
Linux libre here. I ran Trisquel on TTY mode to debug issues, I modprobed the radeon module and KMS switched the resolution to a native one in a laptop.
Starting X with the radeon driver for X messes everything garbled up. The modesetting driver works, but no GL.
At /usr/share/X11/xorg.conf.d/10-radeon.conf:
Bugs may arise, maybe it could be a good idea to disable the Composite extension too.