We’ve done quite a bit of work in this exact field developing our own VJ software built on top of Unity that explicitly supports ProPresenter.
Our first step was to try to support pre-PP 4.2 by developing a “video input device” Unity plug-in, which in developer-terms is referred to as a video digitizer or VDig. The result was pretty bad. It worked ok only in about 640×480 res, and even then wasn’t that great.
It’s important to note that a VDig uses the “old QuickTime”, which is 32-bit and gets translated to 64, and there doesn’t even seem to be any plan in sight to make a VDig use the new QT.
After making a Unity VDig, Syphon released a first beta, and we knew right away it was worth scrapping a couple weeks of work to use it instead. So now, to plug in to ProPresenter, we just use the Syphon Quartz client with PP 4.2+ and it works fine.