Yes, this (or similar) would be quite wonderful! There's currently no way for me to integrate non-Gmail webmail providers with Safari as my default browser.
Ideally, next to the "webmail" checkbox in Alfred prefs, there would be a little dropdown menu with "gmail, hotmail, yahoo, fastmail, custom..." as options... which covers the most popular and would still allow for other providers.
Currently, this is not an issue with Google Chrome, which allows you to set the mailto: handler to anything you want. However, I use the Fastmail web interface in Safari, and while there is a Safari extension to catch mailto links and redirect them to Fastmail, this does not work for clicks generated in other apps. For example, in Alfred with the "webmail" box unchecked, clicking on a contact's email will open a Mail.app compose window, even if Safari is set as the default mail handler in Mail.app preferences. With the "webmail" box checked, Safari handles it correctly, but loads Gmail (which I don't use).
