Unintentional "Shapekey" brush

I just found out that in 2.82, the new Slide Relax brush unintentionally acts as a “basis shapekey” brush. That is, if you paint on a shapekey, it would progressively reveal the shape of the basis shapekey under your stroke. This has been “fixed” as of 2.83 alpha but I find this unintentional feature actually very powerful.
Well, except that it also slides the vertices as this was the original intention of the brush.

Example file to test this:
Download this file and change the extension to .blend
(btw it would be great to enable .blend uploads in this forum)

Open it with Blender 2.82 (not 2.83) and sculpt with the Slide Relax brush :

  • on the yellow side, it just slides the vertices
  • on the green side, it reverts to the basis shapekey

The vertex colors are just indicators and have nothing to do with this behavior. Only the shapekeys cause this effect.

Could such a behavior be re-implemented as a “Shapekey brush” ?

Such a brush was requested on RCS a while ago and got some good upvotes :