Meanwhile, I resorted to calling mpris via DBusInterface directly, that seems to work for now. But your Solution might be more robust – I will test that, as well. Thanks!

version 0.2-3 is uploaded to openrepos.

edit: I could not sleep, so I tested MprisManager. Works brilliantly, but harbour tools don't like "import org.nemomobile.mpris 1.0", so I'm sticking with manual calls for now.