Ok, so you can't. Which svcks. I mean you can fail all of the other daedric quests and still get a shot at doing Hermaeus Mora.
Although I understand the frustration, since A Path of Iron is a long and difficult quest, it's also about roleplay choice. This quest offers gears and weapon to paladins. Molag Bal's quest forces you to have a paladin renounce his vow never to swing a weapon again and then commit suicide. Not very "paladinish"
@Arkngt: IIRC, you can screw up Molag Bal's quest by killing the paladin, instead of having him kill you, which results in you being teleported back to Molag Ball's shrine. Even if you screw up, you will be able to do Hermaeus Mora's quest.