Sending POST request via HTTP using Safari on Mac
Safari is my browser of choice, so I can empathize with you for wanting a native plugin. Fortunately, while there aren't any extensions available, there are quite a few native OSX clients for HTTP/REST end-point testing. I have been using CocoaRestClient, which includes auto-formatting and syntax highlighting for JSON, as you requested. It's open source, lightweight, and is at least actively supported by its developers:
I don't know why you care whether the client is in Safari or not. A restful POST should be able to be executed from any sort of client. You might want to look at RESTClient extension for Firefox http://restclient.net/
Also just Google 'REST client' you should see plenty of other tools available to generate POST's against your service. If you are really would about Safari-specific responses, most good REST tools, should allow you to set the User-Agent header so as to make the request look like it is coming from Safari.