I would love to see this, too. A workaround is to make a project called “Calendar Add-Ons” or something like that, and put the miscellaneous items as tasks there.
If you want to create a calendar from only certain projects (vs. the team calendar, which may list lots of obscure items), you can do an Advanced Search that only includes your “Calendar Add-Ons” project and the other relevant projects. When the search executes it will default to a List View of the results, but you can flip to the Calendar View and you’ll have your own custom calendar. Then favorite the search (click the star to the left of the search name) and rename it (click the carat to the right of the search name), and you’ll have the equivalent of a team or project calendar that only includes certain items.
Asana is starting to consider how to implement start dates now, and I hope one application will be the ability to see ranges as lines or blocks on calendar views.