There is really no good option for this unfortunately, you will probably run into a lot of issues getting some extensions to work and you still won't be able to use some X11 programs such as those using XCB. Implementing X11 actually needs a large number of extensions to really be functional, probably significantly more than Wayland needs. Not that you should implement Wayland; I don't think this method would even work at all with Wayland. It's probably the right choice though if the goal is to just get a few GTK and Motif programs working and you don't really need them to be fast or use OpenGL or anything. Past that you will probably need to implement a full X or Wayland server that implements protocol-level compatibility.
That's really great to see. Just today I was downloading graphics apps in Haiku and looked around for GIMP to no avail. I didn't realize it was such a complex issue and am glad some good people are sorting it out.