was there ever a resolution to this need?
I require the same thing.
The closest I could get was to create a dummy date/time field and create an index in it via a workflow. The problem is then that index shows up in the hover over text and on the calendar when only one item is on a day.
This is being done in a custom (calendar) list with the standard monthly calendar view. For events, not resources however the sorting issue is a main criteria. We need to be able to group a day's service event items by the team which is handling the events. Separate calendars with overlays is insufficient, they need to reside within the same list.