For the Open tickets such as (Super) Off Peak there are Any Permitted and Hull & GWR. The Any Permitted tickets seem only a couple of pounds more. They give extra validity on LNER services which is arguably of value.
As often happens, off peak tickets with different route codes tend to have different time restrictions. If someone who intends to catch a Hull Trains train has a choice between an anytime ticket with one route and a super off peak ticket with a different route, there could be far more than a couple of pounds in it.
If you are booking on a journey planner, they don't exist.
I don't think any retailer encourages people to buy tickets which cannot be used in their entirety, but some do advertise them.
On GWR, you could search for a journey like Hanwell-Goole in the accessible booking tool, select "I need to know how much tickets are before I can decide" and "Off-peak tickets" to get a big list of tickets, and perhaps even buy one.
On EMR, you can go to the results and select "Check for alternative trains" for an even bigger list, this time including U-zone tickets.
On redspottedhanky.com, clicking on "Mixing Desk", and on "show slower routes" (once you reach the results page) gives a list of route codes. These include two HT/GWR options, although the page only shows any tickets if it finds an itinerary.
Surely the off-peak TOC-restricted tickets offered by Hull Trains are basically pointless since either you know roughly when you want to travel (in which case they offer no real benefit over an advance) or you want flexibility, in which case the Hull Trains service out of Kings Cross is just too infrequent to be of any use.
There are some routes (such as Cardiff-Doncaster or Gloucester-Hull) where you can buy a flexible GWR/Hull Trains ticket but not an advance one. Neither ticket has a ✠ on it, so you probably can't get an itinerary via London*, but you don't need an itinerary to buy a flexible ticket.
I assume here that this ticket
is valid via London in the same way that a "Crosscountry & LNR only" London-Sheffield one is valid via Tamworth - if you couldn't go that way you wouldn't be able to use the ticket at all.
* You may be able to do it by finding an itinerary with an overnight stop in London and arranging the cross-London transfer to happen when the Underground is shut. I don't know if that would work and haven't tried it.