What do you mean by access? APIs to program against or fully open sourcing the rendering engine? Because you can mix SwiftUI with a few different rendering frameworks at varying abstraction levels that it itself renders to (AppKit, UIKit, Core Graphics, Metal etc.)
Basically I want an API available to build my own SwiftUI. Definitely not on the Core Graphics level :)
But good point, I actually think AppKit might be a good abstraction level. I'll play with it a bit and see if I can abstract it behind a good component model.