If you're going to detension them anyway, just yank the rim tape off first and use a power drill with a flathead bit. It's less than a second per spoke extra to spin the nipples all the way off at that point. If the slot strips out on a few, finish unscrewing the rest and cut the stripped ones.

Besides, junk spokes come in handy for all sorts of things. If I can find a use for junk nipples I'll be all set.