In version X14 (no release date known yet), the MailMerge class accepts a JSON object as the data source:
The JSON strings should contain an array of objects, very similar to XML files serialized from a DataSet. Relations are built based on nested objects in the hierarchy of the JSON string. The JSON object can contain strings, integers and even images encoded as Base64 strings.
According to this new data source type support, the HTML5 based editor Web.TextControl will also accept a JSON string to fill the merge field drop-downs.
Stay tuned for more!