If I recall correctly Beocenter 9000 has a 12v ‘relay’ output, designed to be used to switch on auxiliary devices. It wouldn’t take much to use this 12V output, reduced to 5V, to trigger the Beolab speakers over powerlink. Perhaps this has been done before.
Alternatively, something like this would work to trigger powerlink speakers with a line-level input: https://shop.oneremote.dk/shop/69116-powerlink-aux-adaptor/4154-powerlink-aux-expander/
EDIT: A second hand Playmaker (or even Beosound Essence) would also work if set to fixed volume output and with IR disabled. Playmaker/Essence both have an auto-sensing 3.5mm aux input (twin phono input for Essence) and powerlink output, albeit you’d need to convert the latter from RJ45 to normal powerlink with adaptor cables.
