Dynamic Form Elements
The ActionForms extend the html:form tags so that they can be populated, and repopulated, from the request.
The ActionForm properties and the tags are tightly coupled, and neither are designed to be used dynamically.
Using the validator form, error checking can be performed by updating the validate.xml and Application resources and restarting the application.
In the Action itself, you would need to add some type of handler to check if the user defined fields were not null. To label them in your email message, you could do some type of "reverse lookup" on the application resources, to get the field labels from the validation messages.
HTH - Ted