rvp /etc/X11/xorg.conf.d/radeon.conf
OK, I have now got this working. So far, this is doable, although the driver is somewhat unstable, with blinking in and out of the browser, as well as tearing when scrolling or hovering the mouse cursor over some text or a menu item. The current situation functions well enough that I feel that I can use this for a period without having to immediately give up in frustration. Tomplay is working as well, with the above caveats. Playback is fine so far, as is the cursor following the music.
And after I just wrote the above, I am starting to get the weird colors and tearing in the browser again, and the cursor has disappeared in Tomplay.