The EU has just started a review of European public procurement rules, and have asked for input on potential changes to the rules. The OSI has responded advocating for changes to dismantle these barriers and ensure Open Source solutions are fairly considered.
And, several countries have laws that favor purchasing the cheapest solutions in the public sector. So a part of the legal argument already exists.
The trick is that Microsoft lobbied enough to make sure that no other viable solution exists, so they are technically the cheapest. Not even kidding. Ms Office has no legal alternative in many countries.