Hi, thank you for your idea! I am not currently aware about Touhou 19 lore since I am yet to play it, but this item seems to fit nicely. I am not sure whether to include common version (it may feel a bit like disabling an already simple to workaround limitation of Lunarians for just a common item, when those are usually near-cosmetic things), but I will think about it.
Using text from Necklace of adaptation is a good idea (thankfully, it is a part of SRD and I am allowed to use it with credit to WotC, which I already do).
One other thing, do you allow to include your idea to the project, under same two licenses I use? I would assume you aren’t against that, but I prefer having an explicit permission.