As a former boarding BM, I see a lot of merit in it. Tack and supplies going missing or being "borrowed" is an ongoing problem in boarding barns, as is available space to groom and tack up, boarders leaving horses cross tied in the aisle for long periods of time, boarders failing to clean up manure and hoof pickings in the aisle so the aisle is a mess from dawn to dusk, etc. In general, maintaing a boarder's tack room is like refereeing a never ending turf war.
So a private tack locker, with cross ties in the stall so everyone grooms and tacks up in their own private space, solves a lot of problems.
Agree that the stalls would have to be at least 12 X 12, 12 X 14 would be better, and that the lockers would have to be horse proof and have a smooth exterior surface. If you can solve those problems, I think it's a great idea.