Currently it is possible to drag and drop a given date from the Orage main window. Unfortunately it is always of the same format, for example 11/10/09. It would be nice to have a box in preferences where to specify the exact DnD format desired, in the style of Orage Clock lines/tips.
As GtkCalendar actually handles the drag and drop totally automatically, this is more like an enhancement request against Gtk. The only way for Orage to do that would be to drop using GtkCalendar and code the main calendar window using private code. This is too much work, so I deny this enhancement request simialarly than Bugs 5763 and 5438 for example.