It's also called the Seven Star Sword based on the Japanese name for the Big Dipper, which is what I'm thinking we should go with.
For now it won't be tagged as long she not yet use it in the game.
That isn't a reason not to. It's a unique object with an actual name which isn't original to Touhou. It doesn't matter that Miko doesn't actually use it.
What waves said. Sword of Hisou tag was out and about as soon as people realized what Tenshi's sword was. However, the comment section isn't a good place for discussing these kind of things so it's at best to punt this at the forums