I'd love to see this too, though I think the goal is to get Video playback working reliably first, which I think is reasonable.
Since Theater View is essentially a Direct3D "game", porting it to OSX (and Linux) is no small task (that's essentially scrapping it and re-writing from the ground-up). So, I don't work for JRiver and I can't say for sure, but I'd guess... No. At least, not soon.
The one thing that gives me a glimmer of hope is that Valve has done some amazing work in making frameworks that allow Direct3D to be more easily ported to OSX and Linux. If they could glom onto some of that (or build a translation layer themselves)... Maybe.
