Facing the same problem, I think the most general route is to create a new template operator which works on the array of objects from the fetch result. Requires a bit of php coding, but the wizards in 3.1beta may help already much.
Maybe an easier way is to use the startdate in the object name pattern in a cleaver way (eq put it first with <start_date>). In your line/full display template override, don't use the objects name attribute for display but only for the sort_by in the fetch function.
Think i'll have to resurrect this thread. I am facing the same problem in that I have a set of projects which need sorting by date, but I can't use the date as the name as I would like to use that name as the URL with the 'nice URL' method.
I notice that v.3.2 has attribute sorting, is this what I am looking for?