OK, replying to my own post (I have some of the best conversations this way ) -- I just called, and the agent had no problem applying the 15% discount to an SFO->MSP itinerary. However, she said it didn't apply to the cheapest available fare (which was $358) -- instead I had to buy up to a $367 fare, and then she was able to use the discount (driving it back down to $320).

I didn't see any mention of lower fares not qualifying in the rules -- has anybody else had any experience with this?

Dan