Thanks athlet1c. Sorry, I hadn't noticed you'd replied. I tracked this down, sort of. Turns out this requires modifying the boot.img/recovery.img and then re-packaging it. Seems somewhat involved. More info here:
http://forum.xda-developers.com/showthread.php?t=443994
I've come up with a solution though. Posted it on XDA and will add a thread here in a sec.
http://forum.xda-developers.com/showthread.php?t=593952