CGS is most convenient, but, sort of like you indicated, there doesn't seem to be a way for a transient pilot to get vetted without first visiting D.C. Here's the procedure:

TSA: Maryland-Three Program

I'd recommend Freeway Airport. It isn't in the FRZ, and a cab ride to the New Carrollton Metro station shouldn't break the bank. I'd be suprised if the FBO there doesn't offer some help with that.