I was able to add a 1-day discounted ticket to my package by calling runDisney for my ToT race and for my 2012 Princess race. I wasn't able to do it for the 2012 marathon weekend half.
Update: YES, you can get a discounted ticket!! (We were quoted a discounted ticket price when we first booked the trip, but weren't sure if we would have time for the park. Last week we changed our mind, called to add the ticket, and were told they don't offer discounts on a 1 day pass...which prompted my original question. I called back today and received the discounted price for a 1 day pass. Cool!! )