Why would you? Even at the height of WWII when the Luftwaffe had destroyed many maltings and the government was forcing breweries to use flaked barley as an energy-saving measure, they typically only used ~8% - and went back to using maize as soon as imports resumed. I'd say that's a pretty good hint that it's not a great idea.
Anyone used Mangrove Jack M15 Empire in a mild? I've not used it but have some in the fridge, it's got a bit of a mixed reputation but that seems to be because people try and use it in imperial stouts and then discover it dies at 8% alcohol. But it's got a reputation for being fruity and adding to mouthfeel, so mild would seem to be its natural home.
As another thought - now would be the perfect time for a partigyle, use most of the first runnings for a big dark beer to be saved for Christmas, and use 10-20% of the first runnings and the second runnings to make a mild. Maybe add some spices to the Christmas beer, could probably use WLP540 for both fermentations (it's a British yeast that came to Rochefort in the 1960s from the Palm yeastbank).