I see, thanks! That technique is widely used and is called a number of things: lattice deformer, grid deformer, warp grid, cage deformer, etc. I agree it is a cool feature, especially when applied to multiple attachments or even the whole skeleton. You're also right that the computational cost is reasonable. Please keep reminding us you want it until we add it! 🙂
Editing the weights of multiple meshes has the challenge of how to select the bone you are painting or adjusting. It could work when other meshes have the same bone.
Grouping attachments so they can be transparent together has some rendering challenges. It's not insurmountable but I think the other features are more interesting to do first.
