I'm especially happy to hear from XviD's Linux userbase here. Glad to help.
Can we expect Avidemux to expose all XviD options in a new release? I've never really used Avidemux because it cannot load custom quantization matrices.
Edit: You know, I'll be missing something like a full-blown Linux MeGUI/VirtualDub equivalent by the time we get AviSynth 3 on Linux. This will be the point when I'm gonna get the craving to do everything (editing, encoding to XviD and x264) on the Linux machine. So please excuse me when I'm (trying) to get a bit pushy here.