I don't know, never tried. 8 weeks before last frost date, for zone 6 that would mean 8 weeks before May 15 - March 15 - I would say don't hold out for miracles. Unless you can artificially heat the ground to at least 65 degrees. Tomatoes need heat and humidity to grow. If you put out plants within the wall they may survive ok, but I wouldn't count on much plant growth until it heats up in May.

I have put my tomatoe plants in the wall o water middle to end of March. I usually set up the walls several weeks in advance to heat up the ground. I transplant the tomatoes during a warm day and hope for the best. One early April we had some snow, and the plants survived just fine. The real question is did I harvest any tomatoes quicker that way? I really cannot say since my garden at this house is terrible for tomatoes. But it is fun to watch them survive the cold weather.

Actually, emlovestogrdn, the wall o'water will come in handy this autumn, which will enable you to ward off frost, and extend the season. I've had tomatos through October, even into early November, withOUT the device you have. Give it a whirl.