Here's a RVA patch for XMMS 1.2.8. It reads RVA2 (volume adjustment data) tags from MP3 files processed by media-sound/normalize and adjusts the volume level accordingly.
Created attachment 18229 [details, diff] The RVA patch
Created attachment 18230 [details] xmms-1.2.8-r2 ebuild with RVA patch
This code is availible as a plugin in media-sound/normalize