Turn a cup while glossing with out touching gloss
Couldn't you just put it upside down and gloss the outside and when it was dry flip it over and touch up if needed?
It isn't entirely clear what you are asking. Is the cup a classic tea cup with a small handle? Could you post a picture of the cup, or tell us a little more about your project so that we can try to offer you some suggestions?
Put you hand in the inside of the cup and turn it from there.
Put the cup over something and let it hang over like a tall can and do the outside first then when dry do the inside