I think it would be rather nice feature to have, to be able to control the default material on a per-project basis.
so that if we wanted we could have a more flashy and obvious dev texture or even keep the style of the dev texture consistent with the art style of the game.
and it would be cool to change it per-project rather than for all of UE4 because one dev texture may be good for one game, but you want another one for another game.
Please consider this addition, perhaps it could even be a plugin?
Is it not an easy thing to change the DefaultMaterial or WorldGridMaterial in the EngineContent/EngineMaterials? You can make any edits or entirely change it, without custom engine versions or even CPP.