[rt-users] Run Checks/Errors on Custom Fields from external database

[Travis Slusher]

How would I best go about checking a custom field for validity from an external database, and display an error if it is not valid?  I want to have Request Tracker check a Custom Field and not create the ticket if it is not valid. 

I am checking against a database so I don't think the Validation property for Custom Fields will help me here.