I've never done something similar in Wayland but judging by how Waybar is setting up it's window I guess it should be doable with backend-winit-wayland
https://docs.rs/winit/latest/winit/#building-windows
EDIT: actually, you might be a bit too early for using winit for that. But from the surrounding discussions it seems that even so, that should still be doable by interacting with wlr-layer-shell directly
EDIT2: hold on, I actually have done something similar in Wayland. Just in C++ and GTK. So if instead of using slint you could use GTK4, I guess https://github.com/wmww/gtk4-layer-shell would be the way to get it working