It's possible and I have a very early E2E test. A modification of the sunshine backend itself to be able to connect to qemu. In the future it's definitely possible to have this directly in qemu but it would require reimplementing the whole sunshine protocol

Follow along https://anvil.fangorn.io/fangorn/sunshine-qemu and https://blog.fangorn.io (there isn't anything here yet)

You need a HW encoder per VM, which means vGPU/SR-IOV for anything multi-tenant. But it would work similar to qemu-rdp (which is a better codebase and really, the successor to SPICE): -display dbus can get the framebuffer to any external process, so a Moonlight-protocol server could attach there. Nobody's done it that I can find.