I thought of something Pop Cap could add to Taco Bandits: the Toxic Taco. There is a small chance a taco will spawn as a Toxic Taco with a purple cast to it, but you don't know it's toxic until you pick it up. If the zombies capture a Toxic Taco, it will counteract with a previously captured taco and bring the taco count down by one. Since a zombie cannot drop it by will, the other plants can just leave that zombie alone, giving the zombies a huge disadvantage. They can't get a fresh taco until that zombie either gets killed or captures the Toxic Taco.
I suppose that another way the Taxic Taco can occur is if a specific plant has the ability to infect the taco while a zombie already has it, but they'll have to do it without killing the zombie in the process.