I want to have remote triggering of flash units on the ground to get still photos.  I just need a signal that syncs to the shutter.  

Has anyone done this?

This is what I did once - because the camera is too wide an angle.  I took a video with a less wide setting and then took a 'snapshot' from there whilst viewing.  On my viewing software I right click the mouse on the screen whilst running the video and the menu gives a snapshot option.  Quite good results.  Maybe worth a try in your case. i.e. record on video with the copter hovering, set your flash units off and see if it works.

Happy shooting.

Yes, that may work, but you cannot sync the flash with the full res. frame (I-frame) in the MPEG-4 stream.  I would like to use the full res. of the camera and synced with the flash.  I am hoping for some "hack" to get the signal from the camera to fire the flash units.