I've done it before, just never checked a bag on this kind of situation, with two different reservations. I called and inquired about this and the lady said that there is nothing they have against me overnighting in the terminal, however she doesn't know that UA will check me through that early. She is the one who told me that if I do not claim my bag they will lock it up until I claim it the next morning but I am apprehensive with that idea. Unless I just go down and just stay outside of the secured area until the next morning, I probably won't sleep much either way...