I think multiple accounts on one ps3 can use "all" downloaded games (not 100% sure though).

I created my account on my friends PS3 and downloaded wipeout HD. Not only was I able to play it, but the other account holders were also able to play it when they logged in to their account (till i removed my account that is ) . So I'd imagine, if you already have wipeout HD, deleting it and downloading the demo under another account will work?