Do you want to give the user of the form the ability to search for posts on individual dates OR find posts that fall within or overlap a range?
The only way around this I suspect is to have a date FAR in to the future and set the Compare Mode to Input overlaps any of the Post Meta range
The problem with this approach is that this end date would then show in the listing for that post. So, to get around this have a second end date field and make that field calculated.
IF the real end date is set, use that in this new field, if it is not set, then use that far away date, and use this field for the end date in the range filter, and not the real one.