[EDIT] double post
http://www.magiclantern.fm/forum/index.php?topic=6600.0
I tried it with DSLR controller (USB)... some cameras its alright.. ie 6D... some cameras it just freezes (7D)... some cameras there is no access to the ML menu (50D). A custom app would be a lot of work.
I am talking about just overlay functions as a fieldmonitor. No external controlling of the camera. The overlays are already very valuable on camera's that have decent video features but lack these overlays.
And I bet it's a lot of work. It's just an idea.