Working within the framework of the existing shoe profile and the inane "threat" of shoe bombings, I'm afraid that the best solution might be to stick with the screeners' judgement, for expediency's sake.

I can think of many good solutions that would work--from a simple laser level set 1" from the floor, so your shoes have a red line on them as you walk past it, and if the soles are taller than the line, the screener can tell you that your shoes need to come off, to the plastic rulers mentioned (they requiring slightly more time per pax buit costing a lot less than the laser levels!) But they require some degree of expenditures, and I'm not sure they speed things up enough to waste money on a process that ought better be discontinued entirely.