It would be amazing if when I clicked on a mailto link it opened Missive rather than the Mail app.
If you're using the native app, it's possible (see screenshot). Or are you talking of opening Missive web version?
I meant the web version but I'll try the OS X one today! Thanks.
also see https://developers.google.com/web/updates/2011/06/Registering-a-custom-protocol-handler to add it to chrome and FF