InterSystems IRIS Business Intelligence - Limiting the source data for a cube

I'm looking for a means to dynamically specify and/or limit the data from the source that goes into a cube at build time.

I have a single data class that defines my source, and as the data within this table/collection grows/changes over time, I would like to be able to control which records actually make it into the indexed cube, but I'm having a hard time (short of defining a stored procedure to use instead of the extent query) for populating my cube indexes.