It appears at the present time we may not be able to use the Apple Audio Unit (AU) plugins with Isadora User Actors. I also use the AU modules a lot for audio routing and frequency analysis. It does appear to be shifting ground with a lot of repatching required for those older projects including those that implemented QuickTime options that are no longer.
That said the new Sound Player routing is brilliant - although some additional functionality to include in-patch output selection and the option to route for loopback input in the Live Capture Settings would be greatly appreciated.
Best wishes
Russell