Can you recommend a good HTTP Debug Recorder and Autoresponder? I usually use fiddler's autoresponder, but it does not enable rules to be set by POST information, only by querystring.
Fiddler can absolutely return automatically generated responses based on the content of a POST body, you simply cannot do it with the UI. Click Rules > Customize Rules, and enter your script inside the OnBeforeRequest handler.
if (oSession.uriContains("postpage.aspx") &&
oSession.utilFindInRequest("mypostname=value"))
{
oSession["x-replywithfile"] = "C:\\fakeresponse.htm";
}