I'm not opposed to the IDEA of a manual safety on a Glock but having actually shot a gun so equipped, the D&L/Cominolli Glock Safety is a mushy, useless ball of suck.
Glock has made at least two versions of a thumb safety model for foreign LE contracts and they made one with a cross bolt/AUG type safety (same type as on the L85 Rifle) for the UK pistol trials that wound up being won by the standard G17 Gen 4.