+1 from me too!
Please add this column as an option in Mp3tag. I see replay_track_gain in the extended tags but it's not in the list to add it to custom columns. Very frustrating.
This field is essential to me for selecting between duplicates automatically with a very large collection. It is a much better indicator of sound quality than bitrate if you don't have time to listen to 100's of tunes. I have often found that a 168kbps file will sound better than a 320kbps file due to recording/ripping quality. The high bitrate is just an attempt to compensate for the terrible sound. VBR files also make it tricky to eliminate low bitrate files.
It is not foolproof of course but it get's closer to judging whether the file was ripped from the original recorded media, or copied several times over. It is based on the likelihood that the louder file will have a greater range of sound in it, therefore less of the original data was removed for compression. The same holds true if the original recording was better.
Volume normalizing level also applys to all audio formats, so no guess work between your mp3's, m4a's, and wma's.
I run all of my files through Media Monkey's volume leveling when they are added to my library. The files with a replay_track_gain higher than +6db are usually lower in quality than the rest of my collection -- it's very noticeable. There are exceptions but it's rare.
Classical music for example, doesn't seem to stick to this rule. 70's music always seems to need a great deal of volume boosting unless you can find a remastered recording.
Hope this helps some folks.