How to make a product attribute of type freeform conditionally required

Hi All,

There is a requirement to make a product attribute conditionally required.
But the catch is that the product attribute which needs to be made conditionally required is of type " FreeForm" and not Enumerated

For EX: Base product with 2 attributes
Product Food
 -Item Name (Enumerated) Say A,B,C
 -Comments (FreeForm)

When Item Name is 'A' , Comments is required (should not be null)
When Item Name is 'B' or 'C' Comments is Optional

I could not find any Constraint Rules that satisfies this condition. There is no condition like
"When the attribute Item Name = A requires the attribute Comments = IS NOT NULL"