Seth can easily AA gief with dp, fst.mp, or fst.mk, no idea why he would ever choose to use stretchy arms. If he has enough time to react and try to fst.hp you, he can just throw a safe boom instead. To answer your question OP, yes it's probably possible if you empty jump into exgh range and Seth attempts to AA you with fst.hp, but if that situation comes up consistently you're playing a terrible seth to begin with. --- "Some things are worth waiting for?!"
Whenever gief jumps his sonic booms, the rival seth will stretch his arms to try and hit gief on his landing. But if Gief immediately does Ex. Hand, it will stuff his stretchy arms.
I don't think Seth has enough time to do any AA against Gief after Gief jumps his Sonic Booms and lands outside of any AA range. Well, except maybe his ultra 1? --- MW3 (Wii): GI-Jew Monster Hunter Tri (Wii): John (US Server)
Nope, Seth can't AA Gief if he's outside maximum fst.mp or dp range (though late forward jumping a lp.boom from full screen gives Seth enough time to dash forward DP). Seth can't use U1 while there's a boom on the screen. You can probably stuff fst.hp with a j.mk actually. --- "Some things are worth waiting for?!"
It's all frames and hitboxes. I assume Gief would be hit while he is landing from a jump but if Seth is too late then you will be able to EX GH before you are hit and blow through it. Not that I'd recommend doing that.