If I recall correctly Glock did a safety prototype in a bid for a british military contract. It was in some horrible place, like a similar size and position to the mag release.
I can only imagine the only reason why they don't have a manual safety option now ala smith and wesson M&P, is it would be admitting defeat to the existing safeties. You know, like the safety on the trigger. Which keeps the trigger from being pulled, unless the trigger is pulled. ;P