After an unanswered post on CF-Talk, too many hours of digging around i've managed to find an MM tech note on how to pass a paramter to Crystal Reports XI. Though not one the most confident inspiring tech notes below is an even briefer description of how to do this:
<cfreport report="c:\\reports\\EmpDept.rpt"> {Employees.Department} IN [#Departments#] </cfreport>