It's no surprise you can buy a generic writable NFC card but it's a pretty cool "hack" that you can turn a hotel card into one because they don't lock the permissions down - presumably because it makes it easier to write different rooms to each card.
Discussion
I am not super knowledgeable about NFC cards but, do you think the cards are somewhat rewritable? Or a hotel clerk needs to register specific nfc cards as keys for a specific room when checking in?
It'll depend on the hotel, but if you can rewrite it from your phone it's definitely rewritable. Hotels don't tend to have top end tech teams.