I can honestly say that I've never had a problem with my fleece pants or
underwear falling down inside my dry suit. Have you actually
experienced this or are you just worried about something you read? If
the latter, I'd say you're worrying about nothing, since not only is
there not any inherent problem, but you can still pull your pants up
from outside the suit and/or use a drawstring waist through the relief
zipper.