I think someone has mentioned that they use either a bumgenius or a fuzzi bunz before. I think it would work anyway because swim nappies are for catching poop not wee.
Yeah an unstuffed pocket would work. Use an old one that you don't want to use as a 'proper nappy' again - coz I vaguely remember being told that the chlorine in the pool would eventually wreck the PUL's waterproofing.