I think that there are enough pipe organs out there in the world that are accessible that this category would flourish nicely.
Unlike pianos and other instruments that I can think of, pipe organs were often made custom to the location, and certainly fitted custom to a location. These organs, especially the older ones, will have unique features, whether there are special sounds to them or how the pipes are presented. There were several great organ manufactures each having their own innovations and characteristic features.
Some are only open or available on certain occasions, such as holidays or concerts. To me, that would be part of the game - finding a time when it was available and go then and post it or visit it. If permission is required or an inquiry about access, then it should be obtained and reported in the post. Some of them are on private property and that should be respected. If a donation is required, that should be put in. Like categories about military posts, a warning shold be put in to respect the wishes of the owner and don't post anything that will get a waymarker in trouble.
In the Boston area alone, there are probably more pipe organs than McDonalds. There are opportunities, like Boston's First Night, where the public goes into churches to hear programs. I also don't feel that there is a need for waymarks to be available 24/7.
So, you got my vote!