Like branhap, I occasionally foul hook a trout when fishing a tandem rig. I catch and carefully release all of my fish. Every fish that I have caught this way does not seem to be hurt any more than lip hooking, except possibly a little more tired and stressed. This is because it is a little harder to bring them to hand as quickly. All seem to revive OK; just may take a few seconds longer.
All that said; I do not like to hook them this way.
The way that I assume this happens is the fish takes a swipe at the lead fly and misses it.
On another note, concerning catch and release, I hardly ever remove the fish from the water, but this is usually not possible if the fish is hooked somewhere other than the lip or jaw.
And, while I am on the subject, I recommend always use barbless hooks if catching and releasing.
Just my 3 cents worth.
Wm