The RDL files are simple XML files. You could create a simple parser that extracts the information you want. If you don't have access to the RDL files right now, you can download them through the SSRS web interface.
Here's an excerpt of the relevant part of an RDL file:
<DataSets>
<DataSet Name="Test">
<Fields>
<Field Name="Testfield">
<DataField>Testfield</DataField>
<rd:TypeName>System.DateTime</rd:TypeName>
</Field>
</Fields>
<Query>
<DataSourceName>MyDataSource</DataSourceName>
<CommandType>StoredProcedure</CommandType>
<CommandText>spMyGreatStoredProcedure</CommandText>
<QueryParameters>
<QueryParameter Name="@param1">
<Value>=Parameters!Param1.Value</Value>
</QueryParameter>
<QueryParameter Name="@param2">
<Value>=Parameters!Param2.Value</Value>
</QueryParameter>
<rd:UseGenericDesigner>true</rd:UseGenericDesigner>
</Query>
</DataSet>
</DataSets>
There are one or more DataSet
blocks, which define one data set of the report. First, all the fields are defined and then the query with its parameters.