I understand. And it get tiresome on the road having to ask for a break when you need it.
As far as a fatigue policy, if you need to call fatigued, call fatigued. If you get pushback contact your supervisor.
If you’re looking for a “work rules” policy, have you submitted a “suggestion for improvement” through the Proper channels? Ie the occurrence reporting link? That would be my suggestion if you see the need for a specific improvement.