Is it possible to have a workflow execute on the condition "if the period on the transaction is in an open period."? I was thinking possibly of utilizing a custom field, on a Bill or Invoice Transaction, that populated a checkbox if the period on that transaction is open or closed? Likely I could use saved search max then follow up with a set field value workflow on a different field, based on saved search result placed in thy dynamic field, but is there a different approach?

Do you need the checkbox to display that information to users? If not, you could just have the workflow run on saved search criteria directly with criteria including "Accounting Period : Closed is false". Be sure that's the criteria you really want, b/c that'll run on transactions where the period is locked in some fashion, but not closed yet.

Good call James - use just need it to display for users. Likely two fields or checkboxes: 1. Identify whether the A/P period is closed, 2. Another identifying whether the entire period is closed.