To come back to the original topic: I have been tinkering with other display managers, and think now that the claim that kscreenlocker_greet uses the DM greeter for unlocking (on Plasma4) is unsubstantiated. When I switch to Ubuntu's LightDM I still get the same unlock dialog. I've built and installed SDDM (into /usr but using dependencies from /opt/local) but haven't yet managed to get beyond its login screen (its PAM scripts must not be appropriate for Ubuntu 14.04).
Things I could still try are starting kscreenlocker_greet with forced raster graphics or a different style so that the unlock dialog shows a keyboard layout switcher (toggling layout back and forth has long been required from time to time in -only- KMail to restore proper keyboard function).
But I could just as well stick with my current fix. I've installed the latest xscreensaver, and configured it to my taste (including using the kclock.kss saver). I've replaced kscreenlocker_greet with the script below, and with that I'm getting more or less the best of both worlds, including a cute legacy look for the unlock dialog :)
@plasma-devel : Can anyone tell me if a screensaver <whatever> exists for Plasma5 that puts up a simple, CPU-cheap clock like kclock does or if not, if kclock.kss can be ported to KF5?
#!/bin/sh
# interface to xscreensaver instead of using KDE's own screenlocking
for a in "$@" ;do
case ${a} in
--immediateLock)
exec xscreensaver-command -lock
;;
esac
done
exec xscreensaver-command -activate
