What would be best is to implement something similar to the way mp3gain or foobar can directly modify the mp3 without re-encoding, avoiding quality loss.
I found this difficult to grasp at first. That is, how can setting replay gain with mp3gain help when the problem is the player is not replay gain aware? If I understand correctly, the explanation is it's adjusting a gain field of every block of audio data, not just writing a
Replay gain tag. I have no technical understanding how audio data is encoded, so I can only accept that on faith. I'm not familiar with the implementation, but obviously this would be lossless as long as there's a way to reverse the adjustment.
I understand this would be an effective way to adjust the replay gain for files to be played in something not replay gain aware. But it seems there are issues with mp3gain writing it's global replay gain adjustment as an APE tag, rather than changing an existing ID3 tag. I'm sure such issues have solutions, but it suggests all this might be a little more complicated than it may seem. So maybe it's just as well this was added to the end of the list. Gives you time to talk Matt into implementing some kind of mp3gain-like solution.
Personally, I don't see much reason to care. In most cases, I would be re-encoding from FLAC (or high bitrate MP3) to MP3 anyway. And where that's not the case, I'm not likely to care about some marginal loss in quality in an already crappy file for which I'm not likely to hear the difference in the car anyway. Also, I assume a mp3gain-like adjustment has no encoding speed advantage, since the whole file has to be written.