Hi Dennis - thank you so much for doing the research on this before I could. I may look in to fixing this in the Debian initramfs. I have done some other things with initramfs in Debian, but I have never needed to change keymappings.
To others who might read this in the future:
If someone does happen to have \ or | in their LUKS passphrase, I suggest adding a LUKS passphrase that replaces \ with < and | with > so you will be able to unlock the device conveniently with a device that handles this key more normally. The other alternative would be to remove this key from your passphrase.
You can add a passphrase to a LUKS volume with the cryptsetup luksAddKey command. There are other places to find more details. Be careful if the data on your LUKS volume is important.