The second link is to Environment.hs, which would have solved my problem, if I'd known what I was looking for. I think I'd have just clicked Back when I saw it was library source code, though.

The third link is http://hackage.haskell.org/package/system-argv0-0.1 - which says "Use this instead of System.Environment.getProgName if you want the full path, and not just the last component."

(note: technically, my code was still the correct solution, since 0install targets Ubuntu 12.04, but yes ideally I would have found that newer versions did support it more simply)