It looks like losetup with cryptoloop did it and there was a dm-xor module for a very old kernel, but can't find anything modern. I'm wading through a ton of obsolete losetup documents.
Something like this:
cryptsetup open --key-file /dev/disk/by-label/randomdatadisk --type plain --cipher xor /dev/disk/by-label/encrypteddisk decrypted
There must be a way to do this, can anyone point me in the right direction?