Lindbergh wasn't under the direct command of the service, but had to take direction if he wanted to fly their airplanes. Tommy McGuire used to sort of send him on errands and, since Lindbergh wanted to maintain relations, he did them. I am not aware of the exact status or privileges the the reps had, but they were there to interact with the active service commanders and personnel.
When I was in the USAF (mid-seventies), we could not tell the tech reps what to DO ... but we could certainly tell them what NOT to do. After all, the property was ours, not theirs. I pretty much left them alone because we were all there to get something done, but I knew guys who would mess with them ... and got away with it.
Perhaps it was the same in WWII, but I don't know for sure.