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First, good question.  Second, no idea how to answer it, but I have a couple quick ideas that might (or might not) point in the right direction.
I know the data isn't available for historical reference to guys from yesteryear, but I Would imagine that O-Swing% on Fangraphs would give some sort of indication that hitters are, as you suggested, forcing the pitcher to come 'up' in the zone.  O-Swing% ought to be high in a pre-cliffjumping version of Wang, and come down a few points when the league figures out collectively that they aren't throwing anything but a low sinker.
LD% and HR/FB don't seem to be all that predictive due to volatility (as you say), but I'm certainly not an expert.  If we had an outbound radar gun in all stadiums, that would probably give you some of the indication you'd need.  Just cross-reference the outbound velocity on groundballs and you'd immediately see the trend indicating that hitters are waiting for a more hittable pitch and wrecking it when they get it.  Of course, trajectory also comes into play on GB's...
I dunno, Matt.  My best idea I suppose would be a comparative analysis of BABIP for a pitcher COMPARED TO ALL OTHER GB's thrown by his teammates for that same year.  That would probably tell you the trend, but I'm pretty sure the data isn't broken out like that anywhere yet.