Bonzai Sports (
www.tribonzai.com) in Falls Church VA just west of DC may rent tri bikes... I know they have a wetsuit rental program.
Mt Airy Bikes (
www.bike123.com) in MT Airy MD, NNW of DC used to do a fairly brisk business renting out aero wheelsets and might do the same with tri bikes. They do offer an extended test ride program but I don't know the details. When my wife and I were looking for tandems several years ago a weekend testride/rental was $50 or $100 bucks...can't see why they wouldn't do the same with a tri bike.
Carl Matson