They can't march in uniform but will wear T-shirts that state their branch of service. And, nicely I think, straight service members who support them will march alongside them.
There's an article about this in the NY Times today. Here's a small excerpt:
Some will accompany a half-ton military vehicle as audio equipment belts out "Taps" and military fight songs to the expected crowd of thousands. They also will hold a 30-foot American flag and a banner with the military crest on it.That sounds so great. Go read it. Good stuff.