Input-method servers, like maliit, need to be known to KWin since KWin
needs to know about virtual keyboards. Virtual keyboards should be shown
as OSD layers, and they are one of the types of windows that actually
should be showable when the lock screen is active.

kwin_wayland --inputmethod /path/to/your/input-server

tries to start the input server. The input-server's window never gets
keyboard focus and is shown on top of all windows except for KWin's
internal clients.