Graeme is correct in noting that you can autorespond using FiddlerScript's utilCreateResponseAndBypassServer() method after matching on any criteria you like.
However, you can also use the AutoResponder itself for this task. From the Fiddler Book
Matching Against Request Bodies
In some cases, a site may use the same request URL for many unrelated operations, specifying the operation desired in the request’s body instead of the URL. You may extend your Match Condition to examine a POST or PUT request’s body by specifying the URLWithBody: prefix for your Match Condition. When this prefix is used, the portion of the string up to the first space character is used as the Match Condition for the request’s URL, while the remainder of the string is used as a Match Condition for the string-representation of the request’s body. For performance reasons, you should specify the URL portion of the Match Condition as narrowly as possible to minimize the number of request bodies that the AutoResponder needs to evaluate. If a request has no body, it will not match any URLWithBody rule.
Your Match Condition may specify the EXACT:, NOT:, and REGEX: prefixes for both the URL and the body. For example:
URLWithBody:ping.php NOT:POST Data I Do Not Care About
Keep in mind that most POSTs from Web Forms encode the body text, so you should ensure that your Match Condition accounts for such encoding. For instance, to match the following POST:
POST http://www.enhanceie.com/sandbox/FileForm.asp HTTP/1.1
Your Match Condition should be:
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