Can’t see this ever happening in this format, too many traditions.
What is more possible is something which relies on the existing Associations playing cross association fixtures. In that way games against your own association would preserve history of those fixtures games against other association would count towards an overall title.
The problem is that neither association is likely to support an outcome where The result is an overall title which could be seen to be superior to their own individual one.
These schools do tradition. Not innovation. They don’t see themselves as owing anything to the broader game or it’s history, that is just not it’s constituency.
So as much as many people would like to see this because there would a number of better more competitive fixtures in the mix, it’s only likely to happen if another Firsts team gets forced into 3rd XV football to join Grammar and High such that the pool of fixtures just gets less and less.
That’s the only way I can see this happening.