Actually, location recording was quite common in the 20's. Okeh, for one, had mobile recording vans which captured some great field recordings - like from New Orleans, St. Louis, and possibly Detroit. Recording broadcasts was uncommon.
Jean Goldkette's ork waxed two titles for Victor at the DAC in 1924. I think the earliest dance-band sides are from 1921 [[Finzel's Arcadia Orchestra) and they were either cut in Det or St. Louis using mobile equipment. The Arcadia ballroom was on Woodward near Stimson.
United Sound opened around 1933, but very few bands recorded in Det until after WWll. Chicago and NYC had the record labels and the studios. Gennett and Paramount [[Chicago/Wisconsin) had studios but they were small potatoes compared to Victor & Okeh. There were recordings made by bands with "Detroit" in their name [[Milt Shaw and his Detroiters) but they were waxed elsewhere, and their link to Det is tenuous, at best.

It's true the independent record derby didn't take off until after WWll, and the reasons for that include material availability but also the AFM relaxing its rules governing recording. Those rules actually changed during the War.