As I said in the general ORAS thread, all the faff surrounding Pikachu and contests seem to indicate that Pikachu will have a role to play in the storyline. In R/S/E, Pikachu was only catchable in the Safari Zone (5% encounter rate), meaning that if you wanted one, you'd have to go for a bit of a detour, spend a fair amount of time in the Safari Zone, and it came fairly late in the game and was slightly underlevelled (though the subsequent voyage across the seas was ideal for training it). In short, Pikachu was kind of an obscure Pokémon in the original Hoenn games.
I think this obscurity won't be transferred to ORAS. It wouldn't be very GameFreak like to go "Hey, you know this one-in-twenty-encounters Pokémon you could find in that optional area back in the woods? It has loads of cool costumes!". They would make sure players were properly introduced to Pikachu in-game before handing out accessories exclusive to it. I think we can even assume players would be given a Pikachu as a gift at some point in the story, to ensure that players would have one available when offered costumes for it. Kinda like X/Y Lucario with Mega Evolution.
The question then is, when would you receive the 'chu? If Battle Tents prevail, and Contests are once again centered in Lilycove (in my personal opinion, this would be a shame, since having them spread all over Hoenn really encouraged travelling), I think it's given that the costumes would be handed out there as well. Even if they are spread across Hoenn, I still find Lilycove's contest hall the most likely place to receive the special costumes.
Then, when would we be given a Pikachu? Not too early on, I suspect, since players would be likely to swap out the electric rodent for something better if many more options presented themselves shortly after. If Pikachu was given in Dewford, it would have fallen into obscurity by the time players reach Lilycove. I suspect this would also mean that New Mauville, Hoenn's resident Electric Pokémon Hideout, could be ruled out.
The way I see it, a gift Pikachu would ideally be presented to the trainer a little time before they reach Lilycove, but not too long before, lest players would box it or otherwise forget it by the time the Costumes could be brought up. The contest hall itself is a plausible location, though I think the "time window" strech between just before gym 5 (Norman) to just before Lilycove. This means it's likely that Pikachu could be available for Winona.
This is where we get on-topic: How would Pikachu affect the gym battle in the Flying gym? It would probably sweep the Gym trainers rather effortlessly, allowing it to rake up EXP before encountering Winona herself. If Volt Switch is Wattson's gift TM, Pikachu could be an excellent pivot even if it was at a lower level than 29, at which point it learns Thunderbolt.
Without Thunderbolt or Volt Switch, though (Wattson could be an ass and give us the TM for Thunder Wave or something instead), I don't see Pikachu performing that well. If GameFreak really want players to keep Pikachu around until Lilycove, I suppose they could let the gift Pikachu hold Light Ball, but I doubt that would be the case. With wild Pikachu being limited to the Safari Zone, the Light Ball would appear unavailable until after Winona.
Though, with or without Light Ball, and even if it appeared as late as Lilycove, I still think Pikachu could make a splash during the water routes (which appear to be reduced somewhat in ORAS, being replaced mostly by islands), the Aqua/Magma hideout, Sootopolis Gym and possibly also Mossdeep gym. And I suppose it could outspeed and OHKO Sidney's Sharpedo/Cacturne. Yay?