Thanks for the input, I ended up reformatting the report to show the necessary data in Group Footers and use a subreport up top to return a shared variable containing the value I need to filter on. Then I'm just suppressing records based on that criteria.
I would tend to agree the best approach would be to have some criteria directly from the db. Unfortunately, I don't have access to write stored procs and the SQL is a bit more complicated than i could get working in a single source statemet for Crystal.