For reference, this conversation was also opened in the VP at eswiki.
In case this helps, the character set in question is the Greek Extended set.
It is available in eswiki (and enwiki) in the character list at the bottom of the wikicode editor. Looks like that function is not present in the visual editor (to the best of my knowledge).
Can this charset be added as "Greek Extended" (or similar name) in the VE special character tool?
Though doing this is technically pretty easy, it's a change that affects all MediaWiki editors probably forever (slowing down the load of this tool by a small amount of time and adding a tiny bit more complexity), so I'd like to make sure everyone agrees that this is the right way to do this. Perhaps someone from the Language-Team team could comment?
I can see this in the non-WP wikis but the name shows as "special-characters-group-greekextended", even in the English wikis.
Also, this is not visible in the Wikieditor, only in the VisualEditor. Not a real problem but I mention it because of the title of this item.