There's no point in using anything other than XOR, because if you did, the results would be the same as using XOR with a different pad. In other words, suppose you have Pad[0] and you use Caesar to encrypt. This is the same as having some other Pad[1] and using XOR to encrypt. Since the pad doesn't matter, so long as it's truly random, the particular method of combining key and plaintext bits also does not matter.
Using a non-standard mixer on the off chance that somebody will obtain the pad, is kind of like hoping your Scooby Doo underwear will serve as that last line of defense against a bullet.
