Setting up OM Processing Constraint, can I create Validation Rule Set with Validation Type = API?

Summary:

Need to set up a processing constraint where RMA creation needs to be constrained if it references an original SO line that is a Final Sale. The Validation Rule Set can't be configured to read data from the original order line, unless I'm able to use Validation Type = API, and create a custom PL/SQL package. The Validation Rule Set page allows me to select Validation Type = API, which then requires populating two fields in the lower left of the page, including 'Label Name' and 'PL/SQL Procedure'. This leads me to think I can create my own package. Is