xlibe VS SDL2X11Emulation

You might be interested in something like Haiku's Xlibe compatibility layer, which got them graphical WINE without a full X11 port.

They are. More than just the drawing routines, quite literally the entire thing directly invokes Be APIs. For instance XQueryPointer just calls directly into the Interface Kit to get the present mouse position, or XSetWMProperties directly modifies the Haiku-native window state, title, etc.

However, that being said, if you wanted to do something similar or equivalent on some other platform, you could probably copy quite a lot of the structure, and then delete the actual implementation and replace it with your own. I restructured things a bunch in the early phases of working on this, and there's a lot of boilerplate code (e.g. macros.c, large parts of the event dispatcher) which you could reuse verbatim; though it's mixed in with the non-boilerplate code.

There were some other projects I ran into that looked like they were more generic, e.g. https://github.com/Abestanis/SDL2X11Emulation (also check the forks of that -- though I'm one of them, I experimented with it a bit before starting on this thing; but it's very incomplete and SDL2 really isn't designed for this), but none anywhere near as advanced as this is now.