All other KDE screensavers make sure that kikbd is switched back to "default" keyboard before locking the screen. Xscreensaver.kss does not do that. As a result if the kikbd was switched into some other keyboard before xscreensaver kiked in, there is no way to type in the password and to unlock the screen (other than logging in using some other means and killing xscreensaver).
I'm not sure I'm catching this; where do the kde screensavers switch the keyboard mode/mapping?
I am not sure who does it and how. But what I know is that when I have my screensaver set to, for example, xrock.xss and I press the lock button on the panel, kikbd switches to default keyboard just before the screenbecomes locked. This does not happen with xcreensaver.kss.
At this point, this is probably not something we're going to look into inhouse.