I have not contacted any churches myself, and so can't vouch for any contacts, but there are some listed on the website under History > Churches. I did contact the Episcopal Diocese, and their contact information is also there for Episcopal churches that no longer exist.

The other idea is to contact the DC city archives. If they have the marriage record, they may have information on where it was.
http://dpw.dc.gov/DC/OS/Services/Access+the+Office+of+Public...