This bug is (was) my fault. I wrote a patch so that replies now get a "d/l code" link, as suggested by liz. However, that patch contained a slight bug, that escaped testing, because my test setup has a black-background theme. The bug should be fixed now -- in a manner quite similar to what you suggested.

Many thanks to petruchio for applying my patches -- both the one introducing the feature, and the two fixing the bug you noticed.

(While I'm at it, there should also be a slightly more helpful message for duplicates. The change doesn't make duplicate detection any more reliable, though, it just makes the message indicate what message you duplicated. Making the duplicate detection more reliable is still on at least two TODO lists. As always, there is no ETA.)