I don't know if you've solved this issue, but I believe there's now a Calculate process in the feature service table.
There is the ability to calculate fields in ArcGIS Online. Here is a link to the documentation that explains calculate fieds: http://doc.arcgis.com/en/arcgis-online/use-maps/work-with-fields.htm#ESRI_SECTION1_28F344E2E80C410A9...