Asdean,
Your second thought about a MAN KAT is much more practical and doable. In fact there are quite a few already converted for expedition use and Action Mobile has a lot of experience with that chassis. And yes, the US military has used them - they also use(d) the Mercedes Unimog which I personally think is the best choice and a 6x6 Mog does indeed have that "wow factor" you require.
The MAN might and probably does get better fuel mileage but if you're really considering any of these seriously, then mileage probably doesn't matter anyway... The MAN has a superb Deutz air cooled diesel and I'd vote for the 17 liter V-12 myself (might as well go big here). U-tube has a couple 1000+HP MAN KAT race trucks (if you can imagine anything that big racing??). It's an awesome display of what a 6 or 8 axle machine can do when pressed hard.
Actually, any of these vehicles would be neat if you were just say 'jumping in for the photo op' - then out to a more comfortable machine to travel to the next destination in...
However, if you truly intend to drive and live in (with) the beast, the Unimog, like Charlie's U-500, is simply the very best all around, do most everything, reasonably practical vehicle you could choose.
I think if you guys come up with a really wild color scheme/graphics combo for your vehicle that would also increase the 'wow' factor. Also remember, most US citizens only identify with the makers of vehicles that are commonly available i.e. GM, Ford, Chrysler, Japanese brands, BMW, Mercedes, Volvo, and some British marques like Jag or the long forgotten MG.
Visually, the MAN KAT and HEMTT are similiar and I promise you only military personnel and EXPO folks here would be able to identify which was which.
Have fun no matter your choice (think about that monster V-12 air cooled diesel - they sorta sound like a jet engine at takeoff.. and with some colorful graphics...that'd be a real big wow for me!):Wow1: