I just got around to testing the latest version of the uBAACH plug-in over UPnP, and for me it is now working. I understand that no plug-ins can work over UPnP with RealTime enabled.
This isn’t an issue for me though. With previous versions of the plug-in and real-time disabled, I would have to wait at least a minute before any sound played using this plug-in over UPnP, with that much of the first track being cut-off. This is no longer the case and the plug-in works fine over UPnP now.
Thanks to the folks behind uBAACH and Audirvana for the fixes.
I will just add though, that I believe the uBAACH filter is still more demanding of resources than the X-Talk Shaper plugin from HomeAudioFidelity, which does the same thing and is priced more affordably.