I've been experimenting with using Haxe macros to implement an MXML-like language, and the repository is here:
https://github.com/BowlerHatLLC/feathersui-xml
However, I've run into some limitations that makes it unlikely that I'll continue with this approach with the full capabilities of MXML. I ran into some issues trying to include Haxe scripts. I could get scripts working in event handlers, but not adding method declarations inside <fx:Script>-style tags.
I could probably create a simpler XML format that is parsed with Haxe macros, though. It probably would have to be fully declarative, and then you'd create separate Haxe classes to interface with it. Kind of a "code-behind" sort of thing.
That doesn't necessarily mean that the full MXML is impossible. However, it would probably need a special compiler that generates Haxe code from the MXML. Since Apache's MXML compiler is open source, that existing code would probably help, but it would probably still be a big job. It would also complicate the build process because you'd need to run the MXML compiler before running the Haxe compiler. A lot of stuff to juggle.
Ultimately, this is something where I'm still deciding what I want to do.