You can't cap or stop caps when Red Pinned or Retreating. If they changed hit the dirt to also do that I think it would be an interesting change.
I think this plus a 5 munitions cost or something would be good steps towards balancing it. They would at least be something to try out. Maybe also Relic could remove suppression immunity and change it to suppression resistance. It would be harder to suppress a squad that has Hit the Dirt! but not impossible.
Making it impossible for squads to stop the cap or decap would make Hit the Dirt! something that you don't
always want to turn on in combat, because even if you win the fight your opponent might get the cap/decap that they wanted and then just move away or retreat without getting chased. Giving it a munitions cost would make it a little more fair for the Ostheer which has to dump munis into PGren grenades against Hit the Dirt! squads if they want to do damage, it would also give players another reason not to pop the ability all the time. Changing suppression immunity to suppression resistance would make it less of a "lol fuck your MG" ability and more of a "well, now I can maybe get some damage in on your machinegun before you suppress me."