All mugs are refillable, but as to whether you can still get a refill on a purchased coffee in Disneyland eateries... one by one the offers seem to have all disappeared.
It's possibly something to do with the move away from amazingly bad US filter coffee* to the Starbuck's inspired espresso style coffees that are popular these days. They can push those out with a charge on each cup which includes a major premium.
(*nb this is not a slight to the USA. If you know anything about the history of coffee in the US you'll know that at several points in history the quality of coffee in the US has declined by degrees. 1849 gold rush, 1900's private rosters take overs, Brazillian coffee crisis, first and second world wars, government supply guarantee deal, the invention of instant coffee, the ongoing south american coffee glut/over supply cycle, the economic choice t plant robusta instead of good varietals --- at each stage the public expectation in quality dropped as economics outweighed taste. Also weird that you could get much better coffee in Canada whose coffee manufacturers went their own way. It's a fascinating history, I'd recommended the book I read, but I forget its name. Meanwhile I'd like a coffee....)
Disney FAQ#275: What is DCA?
DCA stands for Disney Construction Area. All the Cast Members are themed with hard hats and steel toed boots.
Other than the AAA mug offer (which can only be refilled at resturants that have self-serve drink cells, ie: redd rockets, plaza inn, rancho, french market, etc.), the parks have no free refill offers.....
we do sell mugs for coffee, hot chocolate, or sometimes soda....but to get that mug refilled you must purchase a new drink, and there is not a discount.
You can get refills on coffee with a reciept at Market House on the East side of Main Street, but i don't know if they will fill mugs, they are not a part of foods.