Yes, it's probably easy to identify the Haiku-specific bits and start replacing them, but some of the organization of e.g. the Drawables classes is kind of impl

Yes, it's probably easy to identify the Haiku-specific bits and start replacing them, but some of the organization of e.g. the Drawables classes is kind of implicitly built around Haiku APIs, so it might be a little less trivial than it appears at first glance. Still, it's indeed possible, if someone wanted to try it.