Custom invoice validation rules

We'd like to implement some custom Invoice validation rules that trigger holds (or prevent the user in another way to submit the invoice for approval) when the user clicks the 'Validate' button on an AP invoice.

Some of the rules we'd like to implement are:

• Require EXPRESS in payment settlement priority when a Invoice header flexfield is filled with a specific value
• Require the Requester field to be filled when the invoice contains certain accounts in distributions
• Require 'Track as Asset' to be filled when a certain account is used.

In our current setup those invoices are automatically rejected by the workflow system, but it would provide a better user experience if the user can see the errors before submitting it for approval.