I didn't even read #1017842 as the "headline" didn't seem relevant but it does indeed seem to be the same problem. I reinstalled v0.67-2, edited /usr/share/initramfs-tools/scripts/osk-sdl-keyscript to comment-out "cat >/dev/null", rebuilt the initramfs, and rebooted successfully. I saw no signs of the LUKS passphrase being echoed to the screen, with either virtual or physical keyboards.
I guess this can be closed as a duplicate. Thanks for the speedy reply.
