Here's what I did to my luggage rack in order to use big soft motorcycle panniers.
a) shifting the lower rackmounts backwards with two small metal plates
b) fitting two bigger metal plates on the sides of the rack to support the bottom of the panniers and avoid them rubbing against the wheels
c) extending the platform of the rack with, once again, metal bits to allow a rearward position of the panniers and away from my heels.
It is really low tech but it works perfectly for me. Since I did it I rode hundreds of kilometers on rough roads only having once in a while to check of the tightness of the screws, it performed just as I wanted. I noticed no strange feeling from the bike and no damage to the frame as the vibrations are absorbed by the small silver plates.
