I suggest that you create a dummy field (some field you don't use for other reasons) and then copy the track replaygain value to that field as a holder. This way when SONOS eventually deals with track and album replaygain values, you can simply copy the track gain value back with a few mouse clicks rather than rerunning replaygain on all your files. Also, you might want to also do the same thing with track peak and album peak values. Not sure exactly, but those tags may be used by SONOS as well.
edit: I'm surprised SONOS doesn't deal with these correctly. Squeezebox products have an option called smartgain. If I'm playing an entire album it automatically uses album gain value. if I'm playing a playlist with tracks from different albums, the players automatically use track gain.
oh well, if you can use track gain in the future on Sonos or elsewhere it will easy enough to use foobar2000 or dbpoweramp to create new replaygain tags on all your files without reripping. Although, frankly, like you, I prefer album gain most of the time. In fact, I use mp3tag to convert Itunes soundcheck values to replaygain album values so that my IPOD uses album gain info (since it, like SONOS, can only use one volume adjustment).