Datepicker: Allow the use of a function to determine the altField element(s)

I would like it to be possible to use a function to return the altField property for a Datepicker instance.

My use case is that I have many datepickers on a page, a selector is insufficient to return the alternative field.

The code below represents how I would like to implement this enhancement.

    $(".dt-fld").datepicker({
        altField: function(inst){
            return $(this).siblings('.dt-hidden');
        },
        altFormat: 'd/m/yy'
    });