Writer Storm - A Brainstorming Community

I think it's less prudishness and more itchiness, honestly. Nontheless, an Edwardian undergarment might suit your needs better. It's longer than modern butt-covering underpants, but not particularly voluminous. Under the men's uniforms are meant to be particularly tight, I see no reason why it shouldn't fit just fine.

The Wikipedia articles about underwear are really interesting, too.