Most mailers have a place for special instructions, and I think if you just wrote "do not cut," they should be able to accommodate you. It's not such an unusual request, since there are others shooting 35mm panoramic formats and some prefer unmounted transparencies both for enlarging and scanning.
I think whether the slide can be uncut depends on who's service it's going through to get to Dwayne's. I don't know all the details, but I would check this out beforehand. I know if you send to Dwayne's through Wal-mart it's still Dwayne's high quality processing, but you get what you get, no special anything through them.