Most of the fertigation devices will work with the overhead watering system you describe. However, because you already have a large tank I would recommend a device that does not require a pressure vessel. In particular, the Fertigator, Dosematic, Dose-a-tron, Turf Feeding Systems, just to name a few. All of these devices utilize a siphon hose to draw solutions from a tank/drum/bucket. They all should be fine with the Worm tea. However, if you are going to use a product that has more organic solids suspended in the liquid, make sure you have the appropriate level of filtration on your siphon tube. These solids can ruin the injector piston and seals.