I was thinking that a Game Logic could act as the invisible man. Since it sort of acts like units, you could have dynamic waypoints for it, and thus scripted behaviour patterns (it tends to go to escape places, it tends to attack teams and scattered units and it tends to leave tanks alone).
A simple array of units would do for hand-to hand combat. Once the Game Logic is over a certain unit, the unit begins to take damage, panic, shoot, and his team mates will also enter combat behaviour to save him. Even allowfleeing could be used here. A script that assigns these behaviours through probabilities could be implemented.
I'm pretty sure that you can make units fire at a Game Logic but that it will take no damage, though. I'll have to test that this week.
The IR goggles would spoil all the fun, now that I think about it.
All in all, I think it would be fun for a quick, little and different kind of mission of military fantasy.
Just one thought... maybe looking for a single invisible guy will mean too much dead time in the mission just running around... what about a small invisible Seal team? Can you imagine that attack, invisible killers coming out of everywhere at you, the heartbeat sensors going crazy, your own squad shooting each other in a mad attempt to kill what they cannot see?
And what if, after all, High Command were testing how effective this new weapon could be, simulating the escape of the invisible Seal team? What if you were being betrayed by your own people after all?
Mmmmh... I think I'll give it a try myself to see what I come up with. Though it might turn into a ghost story any time... stories are not safe with me, lol. OFP Ghostbusters... I love it
(Just kidding)