If you think about it , the blender painting tool, implemented in inkscape, would be exactly like a bitmap editing feature. Nodes would be like pixels. I am assuming a rectangular grid here, I am not sure what will be the usecase of other kinds of grids while painting.
I am not completely disavowing painting, I think there is lots of potential in the painting idea :).
I think if the gradient mesh can interact with the tweak tool it will give artists some interesting options.
We should definitely push for Types 3, 4 and 5. ( Hopefully a quadrilateral gradient too ? as per paper http://vcg.isti.cnr.it/publications/papers/quadrendering.pdf ).
We should also push for types 6 and 7, if possible after figuring out an unique way to disambiguate things ( colors always show through ? )
BTW, does SVG have an "extension" system where an application can put its own data, that is outside the standard and other applications might not understand ? That can be another way of putting our own stuff in before SVG approves it.