Earlier quoted context omitted.
The Radeon module works on non-accelerated mode with the modesetting driver instead of the xorg Radeon one. At /usr/share/X11/xorg.conf.d/10-radeon.conf: Section "Device" identifier "Radeon" Driver "modesetting" EndSection Bugs may arise, maybe it could be a good idea to disable the Composite extension too.
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).
TTY works well before and after KMS.