WebApi and FetchXml Length

The WebApi supports FetchXML  and executing predefined queries, see https://msdn.microsoft.com/en-us/library/mt607533.aspx.  This works for small to medium sized FetchXML expressions.  However if you have a query that exceeds the max URL length of a browser after encoding the statement, the FetchXML  that gets posted to the WebApi will be truncated.  I'd like to know if there is a supported way to post FetchXml in the body of a request and get results back.  My initial testing looks like this isn't possible, but I'd like confirmation or suggestions on how to complete such a request.

Example:

https://foo.bar.com/api/data/v8.1/opportunities?fetchxml=#Some Long Complex FetchXml statement#