Indeed. The tickets are printed "Any Permitted" and this is the field where any such restriction would be printed, yet the railway industry chooses for there to be none.
To be honest I don't think this is technically possible, as the 16-character limit of the field cannot possibly contain all the TOC exclusions, if GroupSave were to remain a generic discount applicable to a large number of fares. The new-style layout might change that. They could always list all the participating TOCs in the field.
Of course it would be easiest if XC just simply elect to join the scheme, together with all other TOCs in overlapping areas, but I guess there is more chance of a rooster laying a golden egg than that happening. Much easier (and cheaper I suspect) to just deal with each isolated case where a dispute arises than doing something to the current system.
The other solution I could imagine is for all TOCs to adopt the EMT approach, and define a (range of) GroupSave fare(s) for each eligible flow. That way the 16-character field should in theory be able to list all necessary TOC exclusions (or relevant participating TOCs), but I doubt that move would generally be to the passengers' benefit.