This is awesome.
This is important enough I'd donate right now, if (1) there were a donate button (2) it'd fund further development to the point where this was production-ready.
Bringing this from proof-of-concept to mass-market would be a huge amount of work:
1) From the video, latency would need to go way down.
2) Pressure support is important, if not critical.
3) Packaging. To be impactful, this would want to be distributed in Apple's store, and in Debian/Ubuntu repositories (or at least a .deb). Paid on the Apple side is fine (and not contrary to open source -- I'm paying for convenience, and that's a payment I'd gladly make).
Next step would be to go from mirroring to second display. This is less bad than it might seem on the surface; it doesn't involve a whole new X driver, so long as outputting from the current one to a phantom device, and mirroring the phantom display (same thing as if I unplugged one of my monitors; HDMI keeps going out, and I could continue to mirror it).
One possible way to get there is to get the architecture to a place where others can do that work (e.g. leave appropriate places for people to plug in, and appropriate breadcrumbs where others can finish the work).
Disclaimers: Of course, one donation isn't enough to fund further development. I'm not rich, so we're not talking massive donation. Aside from that, by the time you set something up, I'll likely be long gone from this discussion (you don't have my contact info to follow up). But that's also not an abstract comment; I donated to Digimend before, and this would address part of the same major gap. Take the offer more as an indication of the massive value this would provide to at least one person.