Most, if not all, mobos that have a (EATX/EPS12V) 24-pin socket will work just fine off a 20-pin plug. There is a slight potential for extra heat at the contacts, but it's spread over several wires. The 20-pin connector will only go in one way w/o forcing. Avoid adapters or extenders in high-current circuits whenever possible.
Since the latches probably won't line up, there will be no latching. So you might want to tack the connector in place with some hot glue (wipe the outside surfaces of the shells with some alcohol so it will adhere well) - the hot glue will peel right off when the time comes you need to unplug.
.bh.