Have a look on my very own project named Vigie : http://sourceforge.net/projects/vigie/ It's a customer for both SOAP and JSON web service. There is no documentation yet but the source I think is clear enough, and you have as well to navigate thru the SVN repo as I didn't released yet any file bundle. JSon code is in .../Protocols/REST.lua
Oups, even REST layer is using XML documents ... sorry for the confusion.
My main application, Domestik (http://sourceforge.net/projects/domestik/) has switched to JSon as it is more resource conservative vs XML.
I'm using the great dkjson to send figures and alert from my monitors and the code can be found in http://sourceforge.net/p/domestik/code/HEAD/tree/Client/dmkcheckd
(sending code only).
Thanks to Dunkan for pointing this out.