Caveat that I'm using E9.
I doubt this is the approved way of handling it, but I set a couple checkboxes on JobHead as Key Fields in Job Entry. They don't have any impact on pulling in records, and they're always enabled.
I have another checkbox that can always be toggled via a custom button on the screen. Similarly, maybe you could place an unbound text field next to a button, and customize the button to save the contents of the text field to a JobHead field.