So I've also been looking for a way to use an airtag unobtrusively. My cat isn't outdoors, but I have a big house and she's very old and likes to hide. I'm afraid someday she'll be sick or worse and I won't be able to find her, so something like this would make me feel a lot better.
Worst case if it falls off the caller is you just go find it (and come up with a better system).
This is probably the one aspect where Tile has been superior (for my use case). I never noticed the bulk of the Tile tracker on my keyring.
I'm keeping an eye out for less bulky attachments online. Everything I've seen so far emulates Apple's bulky keyring design.
[1] https://support.apple.com/kb/SP840?viewlocale=en_US&locale=e...