For a sec there, I didn't even know what story you were talking about, Bickendan! "Sweet Agony" was supposed to be a bonus story for the fourth Bruno collection, which never got published. The truth is, the story is only partially done, with the first few pages pencilled. I do hope to finish it eventually. Whether I keep it as a bonus story for an upcoming collection (assuming I ever do find a new pulisher) or just publish it online remains to be seen!
As for canonity, dunno if it'd be canon in the Sluggyverse, but it'll prolly be canon in the Brunoverse!