Why not run a virtual x session and run Xmonad inside it?
Wouldn't be lightweight or ..say.. easily GPU accelerated, but should work with some plumbing effort.
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This gives you the advantage of Wayland's modern display stack and no legacy Xorg code while still running a classic X11 window manager.
The main downside is you have to run all X11-compatible apps, it's unable to manage any Wayland-native applications. But, X11 apps aren't going away any time soon, just the display server.