BendSinister: I didn't mind this puzzle -- likely my very first puzzle idea, half-assembled inside my head for a couple of years before publication -- so I guess I spared it from the "archive the things I can no longer readily maintain" operation in June. But I'm not aware of this one piquing the interest of anyone in particular for absolutely ages now and the, er, usual suspects (I really have to find a better phrase) have overwhelmingly found it already over the past 4.25 years.
Thanks to all the solvers. You're awesome. Seriously.
Not least the thoroughly awesome first finders, Brengun1, who provided an appealing dash of symmetry by returning to fetch the cache on my behalf on its last day. :)