Tried that (see above). It is not possible to assign such a tag via the interface.
As a user in our forum noted, searching for emoji in UI is quite difficult, so I like @tobiaseigenās advice on hiding the tag name in the UI (perhaps limited to special places) while using it in the backend.
I would appreciate any pointers to the philosophy and machinery behind emoji to :emoji-name: translation. I do understand, that having pure ASCII markdown might have some benefits, but I would like to have the option to disable these conversions, keeping only the Unicode text.