I don't think it will work either.
You see when the "slime" is injected into the tire, it goes in as a liquid form, so that if there was a puncture, the "slime" would be able to flow directly to the hole (because of the pressure imbalance) and harden to a gel.
If the "slime" is injected without a puncture, it merely coats the inside of the tire forming a the "gel". If a nail were to then puncture the tire, the "slime" in the gel state might not plug it, since it cannot flow to where the hole is.
Kinda hard to follow, but essentially, the slime needs to be injected after the puncture.