I spent ages looking for a solution to this and actually managed to find it about 6 months ago without realising it. I had followed this guide and succeeded in getting a working USB XP installer stick. I'd used it on several 'normal' laptops no problem but had no luck with it on my 08 MacPro or my 06 MacBook Pro. I'd tried Refit but all i got was a blinking cursor. Turns out if I'd been bit more patient with that cursor I would have been rewarded with a working solution but it wasn't until last week when I tried it again on my laptop with a dead superdrive as a last resort that I waited long enough for it to work! Yay! I've not had the same luck with the MacPro however, if I do get anywhere I'll update.
Hope this is of use to someone, apologies if it's been posted already.
Hope this is of use to someone, apologies if it's been posted already.