Lots of songs start on the C List and work their way up. This happened with You Have Killed Me and First of the Gang which both made it to the A list. However, the last two singles stayed on the C.
XfM have also added the song to their playlist but it probably won't get played much. In the eyes of XfM (and NME) Moz is no longer perceived as cool. Radio 2 is the only national station that can now be relied upon to play new Morrissey stuff.
If Boy Happy got some decent airplay (Virgin, R2 A-list etc) I think it could lift ROTT out of the doldrums. It's a pretty good, upbeat song without any really jarring lyrics. However, it will probably suffer the same fate of the last two songs. Still, at least the lack of airplay'll give Moz something to moan about on the UK tour!