it is pretty easy, it takes a lot of steps but it isn't difficult. I made a smooth / relax brush years ago, but I've never though about combining it with shrink wrap which turned up to be great because using only smooth deforms the shape.
To do it manually:
duplicate your obj
apply shrink wrap to your obj and take the dup as reference
Move both operators above the modeling stack
be sure that relax is below shrink op to have the slide effect.
create a weight map
connect the weight map with relax
and to get preserve UV:
apply gator and take the dup as reference
rename old UVs
rename UVs to match the original UVs name so they will be displayed in the viewport
use your brush and paint.
delete weight maps
or you can do it with a few clicks now