My solution to LUKS and keyboard layout problems is to add the passphrase twice. So the same sequence of key presses will be accepted in both US/qwerty layout and whatever you usually use (in my case, DE/qwertz).
If you use more than one keyboard layout you can add more passphrases for them; LUKS supports up to 8 in total, and most people never use more than 2 (one passphrase, one keyfile).
# cryptsetup luksAddKey foobar
Enter any existing passphrase: [type: qwertzqwertz]
--- in another terminal: loadkeys us ; setxkbmap us ---
Enter new passphrase for key slot: [type: qwertzqwertz]
Verify passphrase: [type: qwertzqwertz]
So you add a key, physically typing the same key sequence (qwertzqwertz) three times, but because you changed the keyboard layout after entering the existing passphrase, what LUKS actually sees for the added passphrase is its representation in another layout (qwertyqwerty instead of qwertzqwertz).
Afterwards, it doesn't matter if your Initramfs manages to load your native keyboard layout or not; LUKS will accept either one.