So, me and @d-healey had a look at the Airwindows repo on Github and noticed that there was something weird in the license: while the repo's license was MIT, the source code read "All rights reserved". I e-mailed Chris from Airwindows about this and, incredibly, he replaced all instances of "All rights reserved" with "Airwindows uses the MIT license". Check the latest commit here (warning: when you click on the commit, it may take a long time for the page to load):
This contains all the Airwindows audio open source code - Commits · airwindows/airwindows
GitHub (github.com)
Chris also told me: "You’re welcome to adapt the plugins in whatever way you like". However, as I'm not good at C++, I have no idea how to implement Airwindows in HISE, so I need your help. If you people don't have time to do it, it's okay, you can give me some directions on how to do it myself. Airwindows and Faust inside HISE would make for a killer combo!!!