I have been trying out the Win RT XlsIO Beta. While creating a pivot table I noticed that the pivot table sort order didn't match the source data (not sure why.. possible bug?). I initially couldn't find a way to force a sort of the field either.
I cracked open the assembly with reflector and found that the IPivorField implementation PivotFieldImpl had the desired field. I am now able to sort the field by doing a cast:
((Syncfusion.XlsIO.Implementation.PivotTables.PivotFieldImpl)pivotTable.Fields[0]).SortType = PivotFieldSortType.Ascending;