Hi Jon,
the field mask does exactly what you described. It forces the user to input a value in a specific format.
If you want the value to be auto formatted in DocuWare, it has to be done after the user input by using a workflow "assign data" step.
Live update in a dialog is currently not a feature, but you may request it over at
Uservoice.
Hope this helps.
Greetings from Germany,
Simon H. Hellmann
DocuWare System Consultant