I'd agree with Mareth - can't see a MF nappy working well overnight. There'd probably be compression wicking. A bamboo nappy night be a better choice, particularly if you can boost with hemp.
You could always just pop the longies over a pocket overnight.
Or use the longies during the day, while you wait for the UD. It's certainly been cold enough here today for Iris to be wearing fitteds and wool.