Other possible alternatives (but not tried yet):
1. 1 litre plastic milk carton - cut base off, keep cap screwed on. Take pin and jab some small holes in the plastic around the neck. Bury in growbag, neck down, with base protruding. Water into container... liquid will slowly leak out through pinholes, giving gentle watering/feeding.
2. Variation on the above - bit of soaker hose (or equivalent), one end stoppered with a cork, laid a couple of inches below growbag surface, the free end attached to a makeshift water container.