i.e. a Calendar item (event, appointment, what-have-you) is nothing more than an email with some magic numbers to tell Outlook it's a Calendar item. I think you'd be hard pressed to filter Outlook's "default mail app" based on that (but I won't discount the possibility entirely--I just wouldn't waste my time).

I'm evaluating TUTOS right now, that gfolkert mentioned last week. Needs an improved layout but otherwise looks solid SO FAR. ;)