Really minor point...

I've got title formatting set up to colour tracks in red that don't have replaygain info in them. I've got a track or two that actually get a track replaygain of 0dB and whenever I reload the info for the tracks the replaygain_track_gain tag is gone. I just set the gain to something small to get round it. No hassle but just thought I'd mention it.

mpc files have this problem , when rg=0 nothing is stored...
foobar not have problem.

workaround:
$if($or(%__replaygain_track_peak%,%__replaygain_track_gain%),"color1","color2")

Ahh yes, they were all mpc files. Cheers for that.