Just gave it a little play and it works for me when I point to the luks partition following the Arch Linux wiki.
rd.luks.key=LUKS_PARTITION_UUID=/keys/testkey:UUID=USB_ID_HERE
Note: This is the UUID encrypted partition itself not the mapped device and without the “UUID=”
Oddly enough, doing what you did and following the Dracut man pages didn’t work for me either.
Let me know how you get on.