Last one that I know of (and there could have been one since then) was Molly Powel running her grade horse Pecan at the 1995 NFR.
Barrel racing has become a very specialized sport with specialized bloodlines. It doesn't mean there aren't grade horses out there that are winning, but most the pro barrel racers are going to "increase their odds" of having a winner by buying one that is bred to win, and taking the time to make sure they stay papered.
Heck, I've got 3 geldings (so no breeding that's going to be going on there, LOL) but I always still prefer papers because I like knowing their exact age, proof of ownership, etc. Plus, I also started going to AQHA shows this year and yeah, papers are required!
So this mare you've been looking at, until she PROVES herself in the barrel pen, then do expect to sell her for less (if you do have to sell her down the road) because she is grade. However, if you've got a proven 1D barrel horse, their price usually doesn't change a lot from grade vs registered, because they've proven their ability. Only thing that may hurt resale there is if a person does want a mare they can eventually breed down the road, then they are going to avoid a grade horse.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.