The US army has contracted with a gaming developer for the past 10 years in developing a realistic online game and recruiting tool: "America's Army"
This is a fps with some vehicles, but mostly foot-based combat.
You should touch base with that team and send them an SDK to fool around with. Their terrain has always been lacking. And there are a lot of vehicles that the franchise has always steared clear off... I'm sure this is because of the sheer volume of work demanded by map making. I'm confident they'd be interested in the vastness and accuracy that Outerra offers.