Bah, just an X server (or minimal reimplementation) running in an app, that then renders to its top-level surface.

What I would really like to see is an Android back end implementation for wlroots. Or alternatively, a small Android(/termux) app that just acts as a simple Wayland compositor, that compositors can then use as a back-end. I found an issue about this some time ago, but can't find it anymore.