Never tried that but I don't think it would work. How would it know what Datasource to look in to find the proc?

What I usually do is create a new stored procedure dataset that uses the date proc. Then on the 'Report parameters' dialog, select the 'From Query' radio button under 'Default Values:' - Pick the date proc dataset from the 'Dataset' dropdown and then select the correct Value from the 'Value field' dropdown. Click OK and you should be good to go.