Does Wayland work on non-Linux systems (e.g. *BSD)?
If an application is written for Wayland, is there a way to send its windows to (e.g.) my Mac, like I can with X11 to XQuartz?
Wayland works pretty well on FreeBSD and I know at least wlroots compositors work a bit on OpenBSD (though, I suspect anyone on OpenBSD would prefer to use their homegrown Xenocara). There are Wayland compositors for Mac, the youtuber Brodie Robertson did a good overview of them a few days ago
It depends on what you mean by send. Wayland doesn't have network transparency, there's a bit of a song and dance you have to do to get that working properly. I'm not sure the state of that or of Wayland compositors in general on Mac.
Yes, but still kind of WIP.
Microsoft's WSL2 GUI integration works based on Wayland (and XWayland): https://github.com/microsoft/wslg
Rather than going fully protocol-based (like Waypipe), they used Weston to render to RDP. Using RDP's "remote apps" functionality, practically any platform can render the windows. I think it's a pretty clever solution, one perhaps even better than plain X11 forwarding (which breaks all kinds of things like GPU acceleration).
I don't know if anyone has messed with this enough to get it to work like plain old RemoteApps for macOS/BSD/Windows/Linux, but the technology itself is clearly ready for it.