I have some remastered sets done by Ted Kendall - they sound pretty good actually considering they were lifted from 78's.
The set you'd be most interested in is 'Django Reinhardt - The Classic Early Recordings...'
CD #1 covers 1934-1935 and the liner notes (written by Stan Britt 1992) talk about a small record company named Ultraphone which was on Avenue du Maine in Paris. It doesn't discuss the recording room as such but mentions that the building itself was like an "all-lit hanger...had once operated as an organ factory".
CD #2 covers the early war years, 1938-1939, and recordings from London Decca sessions. The London sessions studios listed were Decca - Upper Thames Street, London EC4. Also Broadhurst Gardens, London NW6 was used. There were also Paris Decca sessions around the same time on CD#3, 1938-1939, but the quality is not as good as the London sessions.
CD #4 is back to some earlier recordings in Paris from 1935-1936 for both Decca and HMV records.
If you contact Stan Britt or Ted Kendall they may have some more info on the rooms used for recording. There doesn't seem to be much in the liner notes. There may be some recording anecdotes on some of the Django websites that a goole search would bring up. There are certainly a lot of Django stories floating around...