Hi, I agree to @n76 - I expect a viewpoint to be a terrestrial one. Nevertheless, why not publish the information that sometimes star parties happen there? The best way to do that seems to add the "description" key to the parking lot, see https://wiki.openstreetmap.org/wiki/Key:description
In that description, you could mention the name and maybe a URL where more info on the star parties is available.
As I am no experienced OSMapper, feel free to suggest better ways to deal with it. 