I was looking at Blackhole on github, and there is a feature request to add AU plugins, may be this is an avenue, as the code is open? I think the old winamp used to take VSTs...Thx for making one!
Regarding listening on the fly with audio hijack, we actually emailed them with the very same idea (they actually have several pieces of software that could potentially decode on the fly with the QUARK plugin) but sadly their software is exclusively in stereo with no solid plans for multi-channel audio. Generally, we would love to build QUARK into all kinds of potential areas such as apps, cars (Tesla is ready made) and smart speakers. If you know any folks into this stuff. We would love to collaborate.