fuffi,
I agree with everything you said, but my solution (and it's handy for other reasons) was to add a tab containing a duplicate "playlist view" dedicated to displaying volume. Once you've updated the files after a scan it's then easy to sort by volume etc.
Volume Column (the playing track is highlighted):
$if(%replaygain_track_gain%,$puts(X,$sub(-1800,$replace(%replaygain_track_gain%,.,)))$puts(VOLUME,$ifequal($get(X),0,0,$ifgreater($get(X),0,$replace(+$substr($num($get(X),4),1,2),+0,+),$replace($substr($num($get(X),5),1,3),-0,-))).$substr($num($get(X),5),4,5) dB)$if(%isplaying%,>>>$get(VOLUME)<<<,$get(VOLUME)))
Track Gain Column
[$if(%isplaying%,>>>%replaygain_track_gain%<<<,%replaygain_track_gain%) ]
Peak Column
[$if(%isplaying%,>>>%replaygain_track_peak_db%<<<,%replaygain_track_peak_db%) ]
You can do the same for Album Gain by substituting %replaygain_track_gain% with %replaygain_album_gain% etc as required.
The syntax for the volume column looks rather convoluted. I can't remember why but it works, and it displays the volume in human-speak instead of ReplayGain's extremely unintuitive "target volume".