How does this work in practise though? You're never told which operators you can use when you go to a ticket office to purchase a rover ticket, and the ticket doesn't show any operator restrictions, yet should you board a train from an operator not participating you will almost certainly be treated as not having a ticket and would (at best) be sold a new ticket. So which is it? It's fine to split hairs on various rules, regulations and technicalities but this doesn't help anyone in the real world if the railway have a different view.
An issue is more likely to occur when an operator explicitly
doesn't participate in a ticket (as used to be the case for many Rangers and Rovers for Grand Central). If there is nothing indicated either way, I don't see why staff would automatically assume it's invalid (well, apart from woeful training or attitude...!).
It's somewhat of a moot point on the East Midlands Day Ranger as the only part of Hull Trains' route where it's valid is between Peterborough and Grantham. As they don't stop at Peterborough, the only context in which that Ranger could be used on HT would be in conjunction with another ticket valid between Peterborough and Stevenage or London. A rather unlikely scenario, I would be have thought.