Um....I thought there already are sand bag shermans in FH2.
And on the topic though:
1) They were not for AT guns, they were for heat rounds from schrecks/fausts
2) Trials proved that they didn't do anything, the faust/schreck tended to just blow the bags away from the tank armour and still have enough power to penetrate the tank. Patton ended up banning the use of them.