Correct me if I'm wrong, but the issue here is if the tip can be paid with the rewards card when using the DDP.
I know you can combine a rewards card and a standard credit card at, say, a gift shop, but I don't know about the DDP.
You might want to call the 407-wdw-dine people and/or look at
http://www.wdwinfo.com/wdwinfo/dining.htm.
Purple & Gold -- LSU fan I take it? Like your coach... Wish we had gotten him back. Go Blue!