I grew hops out of a plastic planter about the size of your barrel. I got 2 OZ the first year and 2.8 oz the second year. I had one bine the first year and 2 the second year. Last Fall, I planted them in the ground, and got 12 shoots this spring! I have a feeling you'll get better results than I did, as you seem to have planned better, but I really don't think you'll ever get much more than a small fraction of the yield you would get if you planted the in the ground. But hey, you have to use what you have!