The only way I see your approach working is if the court imposes such a requirement on MS, which unfortunately isn't likely to happen.I don't see the court imposing this restriction on MS.
But I do see the various governments imposing this REQUIREMENT on MS.
Germany would be a great place to start. They've already decided to go non-MS for their servers.
So, all MS apps used in Germany must have the filter installed and be set to save (by default) all work in the standard, open, format.
Then, anyone wanting to send electronic documents to the German government would require the appropriate filter (this is why it needs to be Open and free (beer)).
It wouldn't matter what word processor you used, they could all read the documents and handle the exchange with 100% accuracy.
Done in this manner, even if MS retained its desktop monopoly, the freedom to use a different suite would be there. As long as that suite had the filters.
As for who handles the standard? I'm starting to prefer an international body. I don't know enough to guess who would do it, though.
Anyone have any suggestions?