If a moveset requires your opponent to be unaware of what your Pokemon is or is capable of doing, then it isn't a viable strategy to begin with. Smogon has never been very supportive of gimmicky movesets. Just because sometimes a moveset will work if you catch your opponent off-guard doesn't mean it deserves a place in any metagame. I think "viable" is misleading, anyway. Viable doesn't necessarily mean "best" or even "good."
Since this has been brought up a few times, what seems to be the assumption implied here is that "non-OU Pokemon" are only worth using if they have some unexpected surprise to throw at players. This is an unfortunate way to look at the less used options in this game. Pokemon shouldn't be selected because they have some surprise gimmick you might be able to catch an uneducated opponent with, they should be selected because they fit a role on the team, so that even once your opponent has a pretty good idea of what you're carrying, the Pokemon is still useful to you. If a Pokemon would be near useless in match 2 of a best of 3 because your opponent has seen it once, it probably wasn't all that "viable" or good to begin with.
I know "gimmick" was never really mentioned by anyone who has posted whining that this is going to make the lesser pokemon less usable, but it sure seems to be implied there. I don't think this makes many pokemon more or less viable(outside of some niche cases like Shedinja, since to be a factor it pretty much requires your opponent to screw up and that'll be even rarer now), it just makes it more important to make sure you're using Pokemon for the right reasons. I think it is important to keep in mind non-OU Pokemon aren't necessarily eating low usage because they are bad and only useful if you have some unexpected curveball to throw, usually they are in those tiers because they fit specific niches that aren't needed on most teams. For the Pokemon who aren't directly outclassed by higher-tier brethren, it's a matter of using them on teams where they fit. This is the difference between fitting an Abomasnow on you stall team and using a FEAR Pokemon. At least to me. Cases like that one will still be there if you look.
Doubles and Triples will probably be emphasized more in this gen than in any other, too, and there's always plenty of opening for fitting in the "right" Pokemon there, even if the BST or usage is lower. That's going to be the fun in all metagames with so many Pokemon available, I think. Lots of choices to fill slots on teams.