Everything is theoretically possible but of course I don't have time to satisfy every personal request. Changing the background requires changing several other things. If you want it, I suggest you to pick up the background you want in the R0 Sysdata (see modded fw 2.20) and then read the tutorial for themes: http://www.rockbox.org/wiki/CustomWPS
The only background that can be found in Sysdata is the one you found. The others are in QMG files. I understand you should be able to change the background of a RB theme by replacing the file matching the theme name in the background folder. I was just wondering if you had the other backgrounds extracted from the OFW or not.
Edit: Also, haven't been able to get any of the Gigabeat themes to work properly. Just saying so others will know.