Holding the slide is not what prevents the hammer from falling. A semi-auto handgun with an exposed trigger that is placed out of battery has its trigger immobilized by mechanical linkage, regardless of the hammer's position.<p>Because of the way Glocks have their lug linkage, many will actually fire even when slightly out of battery. Knocking the slide back 1/10" and holding it there is probably not going to be sufficient to disengage the lug that prevents trigger pull.<p>Even if one could manage to pull the trigger with the gun significantly out of battery, I imagine that this would be a non-event with an exposed hammer because as you say, the hammer would fall onto the slide, not the pin. Still, firing out of battery would seem to be a function of the linkages, not striker-vs-exposed-hammer-ness.