The UN is not a government. It has none of the powers of a government and no binding authority. Even when it's members vote to take action, each country can decide whether or not to support the action.
The OSCE by their own website is really just a political forum in which participants attempt to influence governments on the three areas listed above.
These organizations are not inherently evil but the UN has been identified in WT theology as the wild beast of Revelation. That's the problem. The WTS has always claimed to be politically neutral yet participating in a political forum is not being neutral.
If they were called to testify, although neither organization has the authority to compel anyone to do anything, I could see them giving testimony. But to tell the average rank & file JW that they can't join the YMCA under the threat of disfellowshipping, an organization that started out with religious ties but according to their website does not require any sort of religious declaration to join their gyms, while at the same time supporting a political organization such as the OSCE and working with other religions that according to WTS theology will soon (very soon?) be destroyed smacks of hypocrisy.
You can't possibly compare Paul appealing to Ceasar with this. A good comparision with Paul would be the WTS or any other religion appealing to the Supreme Court. The WTS has made good use of the courts around the world to secure their ability to worship as they wish and there is nothing political about that.
What would happen to the average JW if they joined a community group with a bunch of local churches that sought to change laws and influence politicians? You know what would happen, they would be dragged in front of a JC. How is this any different?