Modifying the request body (no replies)

I have nginx deployed as a proxy server. The client sends a POST request which needs to be modified before forwarding it to the origin server. Is there any module / filter available in nginx. The best I could find was a patch by Maxim as outlined at http://mailman.nginx.org/pipermail/nginx-devel/2013-March/003492.html. However, this appears to be incomplete. Please advise if there is anything that can be used to achieve the modification
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