Is it possible to change the content of a field in a InfoPath form from a workflow in SharePoint. I have tried working with the "Update List Item" action but have been unable to to find a direct way of doing this.
You can always download the xml of the InfoPath form using SPFile and have the workflow modify the contents of the xml file, thus modifying the item. To download the item try something like this
Then once it's download, grab the XElement of the field you want to modify, and change the value. Then convert the XDocument back to a Stream and save it back to the SPFile using SPFile.SaveBinary() method then use SPFile.Update() to save your changes to the SPFile object.