For wlroots-based compositors this is passed down via XCURSOR_THEME and XCURSOR_SIZE just like on X11 although env vars aren't a good fit for configuration.
There was an earlier xcursor-configuration protocol with per-seat live XCursor config which ended up being abandoned. I wouldn't recommend this approach, it's too complicated and roundabout. What exactly are the missing cursor shapes? I'm surprised it's not enough.
