Astropad works, but it's laggy. You're essentially running Steam Home Streaming, which means the host has to capture and encode a video stream then send it over USB to the iPad which renders it like any video stream. Input goes back the other way and is converted into something that makes sense to the host machine.
Best case scenario (wired, host has a solid GPU with hardware video streaming like a modern nVidia card with ShadowPlay) you get 30ms of added lag and some mpeg compression noise (probably more lag if you want to use more than 1080p). In my experience it's much laggier, but i don't have an OS X desktop to test as the host.
The best solution involves a video capture device in the chain. That's basically how iMac target mode works: The iMac has some internal video capture hardware and so it can take HDMI/DP input and render it on screen. I'm sure iPP doesn't have this, but it would be possible to have a separate device that performs the video capture and then feeds the data to the iPP over USB. 2.0 might be workable, but we know it theoretically supports 3.0 so there's plenty of bandwidth. Wireless a/c might work too, but wifi has a less reliable packet stream which will require adding a buffer (and lag). You'd still have an app on the iPP that renders the video stream and sends input back to the host.
That probably sounds like the same system (and it is on the client end), but the difference is that we're no longer doing video capture/encoding/streaming on the host, which is the biggest source of lag (+ wasted CPU/GPU time). The host machine just outputs to display port like it's going to a monitor, and the capture device makes that data into a video stream for the client. Cintiq Companion 2 use this exact system, but like the iMac it has internal capture hardware.
In fact if no one is working on this system already then, you know, maybe I should stop telling people about it and get to work.
