OK, I found a 12V battery pack:

http://cgi.ebay.com/ws/eBayISAPI.dll...35970&tc=photo

It features a selectable voltage output, I wonder if that could work as a basic lamp dimmer?

Also, that 9V 5400mAh battery that I use is a little beefier than I thought, it weighs 13.5 ounces (380g).