I setup a custom column for Replaygain (%REPLAYGAIN_TRACK_GAIN%), and just noticed that this doesn't get read from MPC's (it does show up for other formats). Also doesn't show up in the View->Tags dialog.
The files DO have ReplayGain info, courtesy of FB2K, and visible in that program.
ReplayGain information is not stored inside APEv2 tags, but directly into the MusePack file.
d4n0 requested adding support for MusePack ReplayGain information in the German section of the forum.