So you're not totally against the idea, you just can't think of a good design to implement it right now?
What about the subtractive blend mode? This could be added Very easily. The use case I need it for is for fading out a entire render texture (alphablending and multiply leave artefacts due to float rounding errors) but there is other use cases to, like darkening things or creating worms like 2D destructible terrain with only render textures (no expensive copying) these are only a couple things I can think of right now. I certain there is more.