Linking options to another field selection

I’m trying to figure out how to have a form selection for one field filter or limit the options of another select field. I’m pulling the names and ID’s from the MySQL database fine, but when the user makes a selection from the “parent_subjects” options, I want that choice to constrain the options displayed in the “child_subjects” select box to only the ones related to the previously selected parent_subject_id.

table parent_subjects : id, name
table child_subjects : id, name, parent_subject_id

Is this possible using the where clause in the field options mapping screen? I see options for “IP/username/etc.” but there doesn’t appear to be a way to do this. The plugin does everything else I need really well .. except for this, and I’d prefer not to have to hack on it if I can avoid it.
Thanks!