I have not tried this with a standard SQL statement, only with a Business Object. For the SQL approach, it is not well documented, but you get a return of Metastorm.Runtime.Contracts.IDataContainer, which is not referencable as an array in the way you describe ([]'s).
I would recommend using a Business Object. The design flaw in MBPM is that if you want to use this in a process event, you are forced to add the BO to the process, so it is evaluated every time the folder is accessed. In a form, it should be more optimal, as it is only accesses when the form is loaded or refreshed.
