Blend from Shape Key, but as a Sculpting Brush

Hi. I think this is pretty self explanatory. One feature that Blender is absolutely missing is a brush that would let me "paint" with vertices location data from chosen shape key. Exactly what [Vertex> Blend from Shape Key] does in Edit mode, but as a brush you control with your pen/mouse. For example - Imagine you have a basis Shape Key of a character with closed mouth, now you sculpted a new Shape Key and opened the mouth. It wasn't easy to do it around corners of the mouth, so it was precise work. In the end you zoomed out and realized this opened mouth looks unnatural and part of it, let's say corners of the mouth, need to be smoothly half-reverted to original shape key. It's not really very comfortable to do vertex by vertex selection in edit mode and use Vertex>Blend from Shape Key, entering numerical value for each blend. It would be much easier if I could just paint using my pen and it's pressure.