The only people who currently put any effort into the game engine are the UPBGE developers and it does not really make sense in my opinion to put a formal feature request here. The game engine section would kind of make more sense.
If it is part of a shader, it means all the contained textures would need to be loaded all the time which would have a negative performance impact as all of them would be sent to the GPU.
If you think about a strategy to only send the required textures, I think it is the wrong place to have it. It would need to be outside of the shader for this.
If you’re thinking that using different value levels per color channel will give you more splat textures (in a way that is smooth and looks correct), you are sadly mistaken.
In transition zones (as the red values in the vertex color goes down), you might get lines that show different textures in the middle of it. What you want is the ability to have a custom paint type where you define any number of individual channels given any name you choose, that is the only way that splatting more than 4 textures will really work in a way that one expects.