Someone I know is considering moving to Graphene OS and uses/teaches AppInventor.
Is there anything preventing it from working with Graphene OS?
If it runs / sideloads android apps then it should work...given it is google-averse, some settings may need to be changed, along with allowing for the rendezvous and turn servers....
We don't integrating Google Play Services with App Inventor (by design), so I imagine it should work. As @TIMAI2 mentions you'll probably need to sideload the companion app since you won't have access to Google Play Store.