Steve just posted some code that he uses for this here:
viewtopic.php?f=32&t=1846&start=10#p8746 I haven't looked closely at the code, but I thnk he also has it triggering photos based on distance traveled.
A passthrough for a camera trigger is something I'd also like to see in the official firmware. We'll definitely consider adding it. I'm using a channel directly from my Rx, but I understand Spektrum and SBus users can't do that.
-Max