@geekosaur is correct: this is not possible. XWayland is just an implementation of the X server for Wayland: it can display individual windows, but is not designed to do any kind of higher-level management or positioning (which is the responsibility of the compositor).
Work continues… steadily, but slowly. You can follow the discussion in the dedicated thread, and in the GitHub repo. Currently we’re working on Haskell bindings to wlroots, which are boring and laborious to make. Most of the actual work has been done by @anarchymatt and @bullishOnFunctional.
(This reminds me — there was some discussion on the GitHub about how to handle the XML files which specify the wlroots protocols. When I get more time this afternoon, I should make a post about that in the other thread.)