To produce the the JSON document directly from Teiid, it would require this https://issues.jboss.org/browse/TEIID-2159 , which we are currently targeting for 8.3 release. Right now you would have to manually convert the result from XML to JSON.
On the related note we are also targeting https://issues.jboss.org/browse/TEIID-1854 for 8.3 which will inherently provide Rest based services on any VDB using OData protocol. This also can produce the output in ATOM and JSON formats. I have a initial draft checked in (8.3 Alpha2) on this if you want to give it try. But I have to warn this is work in progress and I have no documents yet.