How to invoke a script in the currently running QGIS instance from the command line?

From the Windows Command Prompt (or Batchfile) I want to find the running QGIS instance with an open project. Afterwards a script (eg. zoom to something) should be automatically started. I've tried it with qgis.bat --code

That works fine, but always starts a new QGIS instance. - I don't want that!

In future I want to register a Windows Protocol for something like link2qgis:layer="layer".featid="123".action=zoomto

When clicking on that url in some third party application, the QGIS instance should be maximized and the map canvas should be zoomed to the feat.

Maybe someone has already done something similar and can help me with that.