Hey, first of all, I love to see the custom rom scene still doing its thing. I think we need to keep fighting to keep our phones as open and as 'ours' as possible. But we, and I mean, a big 'we' as everyone in XDA Developers, Lineage OS developers etc, have as a group a tremendous knowledge in this area.In fact, for the most developed phones, we know almost everything about them. If only everyone could get toghether…
I have vague recollecting of FirefoxOS being like that, where the underlying layer (gonk?) was Android-derived. They certainly didn't go for an Android-compatible app layer though.
With HIDL it should be easier to build a clean abstraction on top of any Android base starting at Oreo.