Myself, I'm thinking a double-piped pocket, with just the opening visible from the front, might have worth. I envision either of two possible locations for such a pocket on the apron... 1.) either right where the sporan would normally hang, such that the sporan conceals the opening, or 2.) just at the bottom edge of where a belt would normally sit (again, to largely have the piping "disappear" or get visually "lost" in the tartan, by the proximity of the belt/buckle.)

Either way, I wouldn't want to see such a pocket be large or capable of carrying anything bulky (no fat wallets), or anything the weight or bulk of which would make the tartan hang funny, the apron hang lopsided, or show ugly buldges. A place to tuck a set of car keys, a slim id wallet, and a $20 bill might not be a bad idea. But then the obvious question: Why not just use the sporan for such?