I clearly see how to disassemble the old-style mags with the metal follower, but the brand new 12-round mags I bought don't have a hole in the floorplate to depress the pin.

What's the trick?

It's the same idea, you have to lift the pin off of the baseplate only the hole on the newer baseplates doesn't go all the way through. The trick with the newer mags is to insert a small screwdriver or pick through the slot in the side of the mag body, hook the spring down low near the baseplate and pull the pin up. Then slide the baseplate off.