It's been a little while, so I don't know if this still exists, but when I stayed at the Grand Californian in '02, you could buy a mug at Whitewater Snacks or from the hotel room mini-bar for about $10 and that would give you unlimited refills at Whitewater Snacks.
I didn't stay at GC, but when I was there about a year ago I ate at Whitewater Snacks a couple of times and did see people with mugs coming in from the pool area and refilling them.