Before we begin, using a workflow I know how to set a field on the initiating record as mandatory.

Goal: Set a field mandatory for only some entries in a sublist.

When creating a sales order, items ordered are added to a sublist. The sales order "Sublist Fields" include fields I want to make mandatory for some items, but not all.  The fields in question reference lists used for option selection. Using a workflow, how do I access the fields on a sublist to make the fields mandatory?

Attempts so far:
--Make the referenced lists mandatory: Not ideal.  The fields aren't mandatory for all items and without default values certain logic breaks when paired items are added to the sales order simultaneously.