No such luck at first but I made the mistake (I think) of trying to install Simple Text in Chrome, not Chrome Canary. I know the apps will sync, but I suspect the new apps must be installed in Canary for the file associations to work (which makes sense).
I then installed Caret in Canary and it does appear in my Finder file associations for a text file. My previously installed apps (Text and Simple Text) don't appear. So I'm closer. ;)
Also of note, I use Chrome with 2 accounts at the same time (Gigaom is a Google Apps shop and of course, I have my personal Gmail account) so I wonder if that's also confusing things. Just thought to pass along in case that's worth the mention. Thanks!