Oh, right -- so I don't believe you can do a JSONP POST request:
http://stackoverflow.com/questions/2699277/post-data-to-jsonp

You'll need to pass the list of desired fields as a GET parameter, instead. Perhaps just like "?fields=online,zone,name", and then you can split it on the commas on the server side.

This may be the source of the issue @bcipolli mentioned to me earlier.