For akuma: Nope by the second parry of his fireball he would have recovered from his fireball lag. Most smart akuma players would sweep b4 you can parry the the third hit of the fireball. Forcing to parry high and low at the sametime. Best thing you can do or i usually do is parry the first 2 hits then block the third. Most of japanese vids i have watched mostly vs against match, players know better than to parry the third hit.
For Ryu: Maybe. I can see what your are talking about. But then again consider how close he is to you where he throws the ex fire ball. Usually what happens is they throw the ex fireball almost a full screen away i parry twice they dash and throw another ex fireball. When they do that it keeps them at a safe distance for the second ex firball. If they do it in a closer range (not full screen) then maybe its possible. But the players here are smart enough when to follow up a parried ex fireball. But if they are really close to you (throw range) and low forward --> ex fireball and you red parry the first hit and blue parry the second hit then most likely you can. I could be wrong about this. I have very little knowledge of frames =P. Maybe slimx or someone who knows for sure can answer that.
As for super fire balls: I dont think you can because its the same reason for akuma. By the time you parried the second or third hit they have full recovered and can make you guess high, lows, or even throw you when you are busy parrying.
I think hugo’s 720 doesnt have any invisible properties for him. So even if you can execute a 720 in the middle of a super fireball it might get stuff by the other hits. Also by that time the opponent should have recovered from his fireball animation and can start the mix ups while you are parrying. I could be wrong about the 720 having no invisible properties. But you can try it out. I am pretty sure Hugo’s saIII can do that. Since it does go threw fireballs, other supers, and normal moves.