Thank you for your detailed reply Niels. I was in two minds about LVM but you have persuaded me! However, it appears it's not in 5.1 so I will wait until 6.0 comes out. I am too new with NetBSD to try -current.
In the meantime there's two wip packages (http://pkgsrc-wip.sf.net/) for LVM if you would like to give it a try.
//Pierre-Philipp