I've got the baby face thing going on, so I'd probably go for the "dirty face" look. When I do dress like a boy, I look like a very young one, so I'd just do the hint of downy peach fuzz and let it go from there.
LOL, think I would go for a fake beard/moustache combo just to try to masculinise it all a bit more. Didn't Kim Bassinger do it in 9 1/2 Weeks and manage to still look good?