Ya know... I love what you're trying to do, but... the WT did not actually "join" the UN... you have to be a country to do that...
The WT became an NGO of the DPI of the UN, which is a big no-no, since they had to accept the priciples of the UN's charter (which has some clauses that clearly are contrary to JW teachings) and make the programs of the UN ("the disgusting thing that causes desolation") known in their publications and report back to the UN about how these efforts were going...
You may say I'm argueing semantics, but there is a big difference between being a member of the UN and being an NGO. The witnesses will be quick to point that out to opposers and that could be the end of the discussion if they don't have any details.
Maybe your shirt could read: "The WT played footsie with the UN", or something along those lines.
If we are not accurate in our critique of the WT what we say will blow up in our face and we do more harm than good to the cause of enlightening others.