An easier option may be to log the POST data before it gets to the server. For web applications, I use Burp Proxy and set Firefox to use it as an HTTP/S proxy, and then I can watch (and mangle) data 'on the wire' in real time.
For making API requests without a browser, SoapUI is very useful and may show similar info. I would bet that you could probably configure SoapUI to connect through Burp as well (just a guess though).