I am also interested in this. Did you ever figure it out?
Sorry about not leaving this as comment, but apparently I don't have permissions.
Actually, I may have stumbled on the solution.
I noticed that when you download the report definition it has a field called d:CustomReportXml. In my custom reports this field contains XML code.
<d:CustomReportXml><CustomReport><Language>1033</Language><Query><fetch version="1.0" output-format="xml-platform" mapping="logical" distinct="false"><entity name="appointment"><attribute name="createdby" alias="createdby" /><attribute name="createdon" alias="createdon" /></entity></fetch></Query><Groupings /><Columns><Column ID="createdby0" Field="createdby" Width="100" /><Column ID="createdon0" Field="createdon" Width="100" AddRawValueColumn="true" /></Columns><TableLayout Display="Normal" /></CustomReport></d:CustomReportXml>
inside the XML is a fetch tag
<fetch version="1.0" output-format="xml-platform" mapping="logical" distinct="false"><entity name="appointment"><attribute name="createdby" alias="createdby" /><attribute name="createdon" alias="createdon" /></entity></fetch>
I have been able to parse this out and pass it back to dynamics via the api and get back results. I am using http://www.xrmtoolbox.com/ to test the results and so far things look promising.
I can also add a filter to the fetchXML so I don't end up with too much data.
I hope it helps you.