One can pass additional arguments to KCMs using "--args" which was not exposed through the QML API yet.
Adding a new default parameter on QML side is compatible with one argument invocations.
KQCAddons.KCMShell.openSystemSettings("kcm_keyboard", ["--tab=layouts"]);opens SystemSettings keyboard KCM on "layouts" page
Opening printer KCM (invocation without argument) from applet still works.