Linux Tablet: Is there a way to enter a password with onscreen keyboard to unlock a LUKS crypto device during bootup?

I just installed Linux (Ubuntu, but it does not really matter) on an x86-based tablet. I also encrypted a partition which is mounted during bootup. Of course, one usually needs to enter the password to unlock the partition during bootup. **Is there a way to unlock (cryptsetup luksOpen) the partition with onscreen keyboard (touchscreen only, no physical keyboard present!) *before* the first user logs in?** The onscreen keyboard (Onboard) works very well during login (in gdm or other display manager) and later on. I do realise X has not started at the normal time /etc/init.d/boot.crypto (or newer systemd) interactively requests passwords according to /etc/crypttab, so the question could mean, is there a way to defer unlocking until X has started (but before a user logs in). PS: The process usually showing the password prompt is plymouth which asks for a password using . It does use a graphical display mode but I think not X?! A lightdm integration would be the other alternative (with onscreen keyboard readily available).