No they can't, bringing them back would be making assumptions about peoples character, generally looked upon as a no-no. I killed ALL of them, I would be pretty pissed if OB just up and decided "Ok they are all alive and Revan is lightside" Thats defeating the purpose of a ROLE-PLAYING GAME.
BioWare did that in BGII with Minsc making a return, personally it annoyed me, I hate him. But at least the brought Edwin back too so all's good (in the first one your forced to take one side and kill the other).
That still doesnt explain whyt Calo should come back....
And I already said Cassus is presumed dead, your just repeating me, I never put that out as fact.
Anyway....