Thanks, that concurs with my own analysis here.
> 1) make the password prompt single line
impractical without losing information the user may need in order to correctly identify the volume they're being prompted for the password on
> 2) clear the whole screen instead of clearing a single line before writing the password prompt.
but I guess that means any other messages will be lost, some of which might be important - for instance, what if the user types a passphrase, the disk fails to unlock and cryptsetup displays an error message then reprompts. I guess the user will only see the prompt?
> 3) append the bullets instead of the line
This seems pretty much ideal to me. Otherwise, could the details plugin parse the prompt for newlines and only redraw the bit after the last line? Or is that jumping too many abstraction layers?