Hi Kieren,
It's one of the reasons I first suggested the "anchors" idea over 13 years ago (see https://lists.gnu.org/archive/html/lilypond-user/2009-07/msg00498.html); when I picked up the idea again (search for "addAt"), that thread eventually revealed the edition-engraver, and may have led eventually to David's \after function (?).
The thread on "addAt" was in 2014 (https://lists.gnu.org/archive/html/lilypond-user/2014-01/msg00660.html).
When I was working on making \after "official", I searched for the earlist version (basically I wanted to take stock of the various versions that came up over the years to decide which elements to use); the first incarnation I could find was from 2012: https://lists.gnu.org/archive/html/lilypond-devel/2012-09/msg00342.html
Lukas
