I would have expected some relation to clave/key (but maybe I'm just being naive)

I was concentrating on the secondary meaning. Latin clavo is from clavus 'nail' as opposed to clavis 'key' (whence our clef).

For naive, I used the HTML entity iuml between an ampersand and a semi-colon, but as noticed by others in the bugs thread, something's wrong with the forum software configuration, and no diacritics or Unicode.

Ceci n'est pas un seing.