I've done a backport of the 9.3 JSON enhancements, as a 9.2 extension, or rather as much of them as it's reasonably possible to backport. There are some caveats to using this extension, so please read the warnings carefully before you use it.
If you use PGXN you can get it from there, or it can be got from the public git repository at bitbucket.
Thanks to David Wheeler for prodding me to finish this up.
Enjoy.