Sure you can. There's a difference between documenting a protocol and documentation the keys. The keys don't have to be documented, only the protocol (exchange of information) and the key format. Once you have that, you license the actual keys. That's how every encrypted protocol works in the open standards world.
You're not looking at it correctly. Key exchange is not the problem.
Key exchange is a way for TWO parties to exchange keys with each other over a potentially insecure network so that they can communicate securely with each other.
It is not a way for one party to restrict how the other can use content.
I'll repeat. If we have an open DRM standard that anyone can implement, what stops me doing this:
Browser: I want video B please
Server: Video B is encrypted with this key and is only to be viewed once
Browser: Ok, I'll only let them play it once
Browser: Hehe, I'll just save this in an unencrypted form.
If there's anything that prevents you doing that, then it's not open.
All of the current proposals (including the major one from Netflix, Google and Microsoft) effectively block support for open-source browsers for this reason.
Others use a form of closed-source plugin.
The W3C disagrees. You need to conform to HTTP standards, HTML, DOM, CSS standards, you need to implement a number of ratified standards or else you're not a Web browser
All of these standards are openly available and do not require the developer of a browser to sign up to anything or restrict how their browser works.
I think you'll find that the W3C agrees with me entirely on this matter. They do not see how DRM can ever become part of HTML5 video.