Which would be obvious, flout the law, and allow for legal action to be taken.

The company itself has to use the format for live message exchange, have to publish a full working protocol document.

With OOXML Microsoft aren't required to be able to read & write a files correctly when it's written to the protocol. As I envisage it the requirement would be to open the protocol actually used "on the wire"; again with Word Microsoft don't use OOXML as primary.

I think it's workable except perhaps in requiring political will. It's an internet age equivalent of allowing third-party car parts manufacture.

Oh yeah. They'll probably try that. That will either be defeated with tons of sweat as it largely was in products like OpenOffice or the legal recommendation modified to attempt to deal with it. It might also be handled in the courts if the law includes language that specifically instructs to attempt simple, readable protocols to "facilitate interoperability." Obfuscations could be argued as the opposite of that in court.