Really, it's a lot easier to change weight on a barbell than it is a dumbbell and I'm thinking the quickest type of dumbbell is not going to be all that secure.
Never the less you are hot on the trail of my laziness. In my gym I have 3 barbells and a trap bar. The trap bar and the barbell that is alway on the floor are both always loaded with starting weight. One bar is always in the cage ready for squats and the other is on the outside of the cage ready for anything.
You probably missed the thread a few years ago, but I basically went into orgasmic seizures when I got my rolling plate holder (that's right, rolling fucking plate holder). All I need now is one of those lever bars for lifting a bar off the floor to load and unload plates.