No progress here on the front with radeon driver in non framebuffer mode, but I have succeeded with non framebuffered vesa and with framebuffered radeonfb VTs: Vesa driver does not exhibit the same problem but is not accelerated and defaults on this PC to 1280x1024, which is a 5/4 format that does not fit very well a 4/3 screen. So for the present being, VTs are framebuffered (grub2 loads vbe and radonfb, radeonfb is uncommented in /etc/modprobe.d/framebuffer_blacklist.conf). X is not framebufferd though: radeon module is still loaded too.
