You and I will find out at the same time about rhubarb in a half-wine barrel...
I misread something in John Jeavons' How to Grow More Vegetables
last year, and planted four (yes, FOUR!) rhubarb plants in one square foot of my Square Foot Garden. (I use a combination of Jeavons' and Bartholomew's methods.)
Well, of course the rhubarb plants didn't have enough room to keep a bird alive, but they definitely thrived, at the expense of almost everything else in their half of the SFG.
A couple of weeks ago, we moved the rhubarbs into their own containers. The three smaller plants went into one-gallon pots, just as a temporary measure, and the largest of the four went into a half-wine barrel-sized container. It's doing quite well and has begun to put out new leaves, so it seems to be very happy!
I planted it in a combination of 50% bought organic potting soil (left over from last year) and 50% my own compost, so there was plenty of air and plenty of water retention in the container.
Go ahead and try it!
Sunset Zone 17, USDA Zone 9