It seems to me that, as I've slowed down (drastically) in answering these suggestions, players have slowed down in submitting them. Which leads to the unsettling possibility that any effort I now make to deal with the backlog might set off a new torrent of requests. I don't really want to set that off - I would rather unset it off.
Anyway, to answer your question, smaug, unset is listed by quite a few dictionaries as an adjective, meaning "not set". But some dictionaries note that it can also be a verb: "Put out of place or position; undo or cancel the setting of (anything)" (Shorter Oxford). I think of unsetting an alarm, for example.
But is it common? In written texts, it doesn't seem to be used very frequently. The New York Times has just one example in the past 12 months: "The only jarring note is that unoccupied tables are left unset save for napkins — as a result, they look positively naked." The Fairfax newspapers in Australia had a handful of examples from the past few years. An example: "...a gift shop which has jewellery, including set and unset gemstones." The Times of London has rather more examples, 23 from around 8 years, although some of these are in the solutions to Polygon, a Chihuahua-type puzzle.
Probably the biggest number of instances I saw were to do with food - jellies, whites of eggs, creme brulees, etc.
In summary, the usage evidence is not strongly in favour of making unset common. However, its meaning is certainly quite clear whenever it is used. That fact, plus the variety of contexts in which it can appear, have persuaded me that it ought to be common.
