Experimenting on the image I think I am able to say that the "plymouth ask-for-password --prompt FOO" functionality is not working. If you envoke /lib/cryptsetup/askpass FOO and type a password you will get the password string as text on stdout, this does not occur with plymouth's ask-for-password, the prompt is shown and can be interacted but the client simply exits 1 rather then emitting the typed text.
Experimenting on the image I think I am able to say that the "plymouth ask-for-password --prompt FOO" functionality is not working. If you envoke /lib/cryptsetup /askpass FOO and type a password you will get the password string as text on stdout, this does not occur with plymouth's ask-for-password, the prompt is shown and can be interacted but the client simply exits 1 rather then emitting the typed text.