looks like all their mounts want something round to attach to, huh? are any small enough to fit to a seat stay? I take it you do not use a rear rack? if you did have a rear rack, I've seen riders attach PVC tubing to them & then attach their light or camera to the pvc tubing
this person did that with a front rack
BikeHacks: Rack
do they make a helmet mount?
also just got reminded of
this thread
and
these things