I wanted to use a banded mission coupling over a galvanized stub. It would be placed over the threads of the pipe. Is that a bad idea?

I've done it with no problem..

Right or wrong....I'd probably load the threads with silicone. Belt and suspenders kinda thing.

load with silicon? please explain?

I figure you would be changing the diameter of the pipe then? creating an irregular size, not a proper seal?

I have done the silicone and no-hub thing before. Use a quality 100% silicone and smear it all over the threads so that they are filled with silicone. while it is still wet, just attach the no-hub and torque it down. In case of a backup, the threads won't allow water to seep out since the silicone is sealing them.

When I've had to clamp over threads, I rub a little silicon caulk on the threads.

I wonder, if you attach a no hub coupling to a threaded end of a pipe then torque it down, will the metal threads bite into the rubber sleeve hard enough to cut into it?

If it was new threading it might, but most instances the threads are round or nonexistent if they need a fernco.

They will "cut into" it, but since the rubber is NOT as thick as the threads are deep, it will not fill the groove, (and if it did, it would be because the thread has cut all the way through the rubber which would not be a good thing). As a practical matter, and due to the dynamics of a "double trap" they will seldom leak on a horizontal connecton with a gravity drain.