Excessive Motorsports in Manassas VA is very Mustang friendly. They sponsor the local Mustang club (Northern Virginia Mustang Club, check it out!), and the club hosts dyno days there every so often. Lots of club members rave about their service. I've only had a couple dyno runs there, but they seem very friendly and on the level. I plan on taking mine there when I get the CAI put in.
The club also has a couple other sponsors, but I honestly don't know much about 'em.
It's about an hour drive south but there is a place in Richmond that I believe will do dyno's. They mostly just sell cars but I know that they also do everything else as well. Just go to the site and then click services at the top to get the info.