> I find myself wondering what happens (or would happen, I suppose) if the people being poked at were told a) before; b) during; and/or c) after the experiment about this effect on their thoughts. Could they then successfully get past the negative to either a place of balanced neutrality or positive view?

(nodnod)
I think about this a lot. In my own experience I've found it possible to:
a) observe my own negative/positive biases about various groups (and I have plenty!)
b) file alerts in my mind that lead to reactions like "Hm... I find so-and-so has (trait), but I also note that so-and-so is an X, and I've noted in the past that I tend to read Xes as more (trait) than they actually are, so I'll assume a fair portion of that is my own bias."

Sometimes with the bonus result that the bias itself fades over time, since I'm not constantly reinforcing it with bogus "examples." (Sometimes not, though... there are biases of this sort I've carried around since my teens and simply have to keep compensating for.)

So insofar as knowing things like this helps with step a, I'd say that yes, it's possible. (But slow, and takes work.)