reinrab, that is a good discovery. A reminder on how important it is to do a thorough inspection with our trailers when possible.
You probably have more than just a nut in the drum.....it is probably a stud. The brake shoes ride very close to the drum, so there is probably not much chance of it getting between the shoe and drum, but it would still be in there bouncing around.....not a good thing.
Like Neil said, it might be best to jack up the trailer, remove the drum, and replace the stud. You will probably have to get a new stud, as it is no telling what the old 1 looks like after the ride it has already had. Any nearby trailer shop should have that part.
If you are uncomfortably doing that kind of work yourself, then maybe you can get 1 of the local trailer mechanics to come to your house. He will probably need to check out the wiring, magnet, shoes, and drum for damage.
I had the welds break on that square flange on a long trip last summer.
I stopped at rest area and found that hub hot, hot. At 1st I thought it was the bearing.....the wheel would barely turn. But after backing off the brakes, the wheel turned freely, and I continued the trip. I didn't realize that the flange had broken loose until my local trailer shop showed me when I got back from the trip. I should have seen the problem when I was under the trailer backing off the brakes....bad boy.