Most mo-track is done in post, true. But I need to do this live.

Are there any key boxes that handle motion tracking live, or at least in RT?

I have a huge greenscreen and I need to move the camera around the actors, not keep it locked down.

Not certain why you need motion tracking for a chromakey job but then I don't entirely understand what you are attempting to do.

There are several hardware boxes that will do like chromakeying for you.

Ultimatte may be the best known. Panasonic makes one too.

Almost all require a RGB feed though.

I found several with a quick search on ebay.

there are several product that are able to give you the camera position in real time.
The fact is if you use chroma-key, that means your background is generated by computer, so the 3D apps is supposed to receive the feed from the motion tracking system to simulate the camera move (eventually including zoom and focus changes)
many application are able to do this directly or via a plugin.
To make it short, this is a very expensive and complex setup.
you would probably rent this service instead building our own system.