It finally stopped raining so this morning I had chance to try a real test flight. I learned two things:

1. It works! Well .. I suppose I knew it works, but I'd not actually tried it outside before.
2. It drops frames!! This is more of a concern and, as yet, I'm not what the problem is. Connected to the AppleTV over Airplay, you can see the iPhone and Goggles drop frames fairly regularly (1 .. 2 .. 3 .. drop .. 1 .. 2 .. 3 .. drop). When not connected, the iPhone video appears just fine.

So while it's dropping frames, its not really usable for flying. I need to do more investigation to see if I can work out why this is happening; is it a limitation of the network, Airplay Mirroring, the older iPhone I'm using, or something else entirely.