First rationality diary I can find on LW: 2012-05-14. This one already says "instrumental". It doesn't say it was your idea and seems to imply it was someone else's (which might, among several possibilities, indicate that it isn't actually descended from the original suggestion you have in mind here).
This is the public group instrumental rationality diary for October 16-31.
Thanks to cata for starting the Group Rationality Diary posts, and to commenters for participating.
Previous diary: October 1-15
Rationality diaries archive