I think it will be purely incidental if the reveal is only in AMOL (which wouldn't stop them from enjoying that, but I really doubt they took that much into account at all).

Brandon and Harriet picked the best scene for the revelation in RJ's outline, before Brandon even began to work on the book, and long before he even suspected the book would be split, not even in two but in three.

It will be added to a scene which was in the outline, so unless that scene can happen at any time (which is quite doubftul), the revelation will come when that scene shows up on the timeline and that's all.

I'll be surprised if the reveal isn't in TOM, actually. The "third act" seems like a definitely odd place for scenes in which the killer will either reveal to someone he/she killed Asmodean, or muse about doing that. It's also a revelation that will have no impact on the plot, as Jordan didn't even include that in the outline, and when he changed his mind and decided he would reveal the killer, he didn't even know if he could fit that in the book or would explain the events only on his blog.... Of course, it will be tacked into another scene, so that won't even be the main matter the scene deals with. The "no impact on the plot" aspect makes it most unlikely anyone in the Light will be the recipent of the revelation (and it's already most unlikely the killer isn't a Forsaken, and even less likely the killer is aligned with the Light). For instance, I seriously doubt Brandon/Harriet would have decided to make Rand aware of who killed Asmodean when RJ didn't plan to ever have Rand learn the truth. That would go against RJ's intent. In the most unlikely case a lightsider killed Asmodean, this will be revealed strictly through inner thoughts.

If Graendal is the killer, this will most likely be revealed in a scene with Aran'gar than happens before she died in TGS. The context would be excellent, in many possible ways. If Graendal/Aran'gar discuss Demandred/Mesaana, Graendal may muse on taking out another Forsaken. The same if what Aran'gar brings up makes Graendal think on the risks in their alliance is becoming dangerous (eg: Graendal thinks Mesaana/Demandred have toppled Aran'gar, and she may be next.. especially after she witnessed their humiliation recently) she might reflect on how she cleaned up the mess and the loose ends of Lanfear's catastrophic "alliance" in TFOH. She might also think on Moridin's promise of reward, that Aran'gar mustn't learn about this, and muses she might have to take out Aran'gar once she's done doing to Rand what Moridin wanted She could also have discovered that Aran'gar is meddling with her DF, and decide their alliance is over (it would be rich if Graendal killed Aran'gar in that scene!) And so on.

If the killer is dead (eg: Sammael), the two characters most likely to know who killed Asmodean, and bring it up, are Moridin/Shaidar Haran.

In any case, it doesn't sound very likely there will be a lot of scenes like this in the third act. AMOL is likely to be the most fast paced/action oriented of all three books.