no, the answer is as follows:

instead of trying to add a second filter to the Year parent group, build the constraining expression into in the column:

=sum(iif(Fields!MonNo.Value < datepart(dateinterval.Month, today), Fields!SentLeads.Value, 0))

this returns only those rows having month numbers less than the month number of the current year. So simple, yet elusive because of where it needs to be placed.