I am thinking about it more and more and T-shirts do sound best to me! It will look amazing, they will not be for sale, but are a gift for contributing to the costs of the party and raise money for charity. If we play our cards right, we could have some newspaper coverage as well, cause in a sense we are organizing a big charity party, with all the proceeds (minus costs) will go to charity.
We could even call it the drinking for holland charity party. Only people contributing (wearing a tshirt) are welcome. Even if it does rain, you could wear it over your raingear, it will get wet, but who cares. The most important thing is that you are recognisable as a group and freeloaders will think twice about crashing the party. Also to keep a check on who is coming, we can ask people to pick up their shirt personally, sending out gives people time to copy it and that is something we have to prevent.