how to design a cover crop for large containers

Due to deer and rabbit pressure, I am mostly growing my vegetables in large plant pots and half barrels currently. The trouble is keeping the fertility and soil tilth up without disturbing the soil.

The plant pots are about 1.4 sq ft and the half barrels are 2.6 sq ft.

One of the barrels got a fair bit of finished compost added to it this spring, whereas the one beside it, got almost none. The difference in the vigor of the beans I planted is really noticeable, with the compost making a huge difference. The problem is that the compost is a lot of work and materials to make and tends to shrink down a lot. What I need to do is be improving the soil in situ, which I admit is one of the reasons I grew the beans in the pots in the first place.  

Rather than hoping for a few beans from the poor soil pot, should I be chop and dropping a percentage of the plants when they start to bloom? I was told once by an organic gardener that, "it isn't a soil building crop if you take any product from it. The whole plant must go to soil building." Does this sound right to all you permies?

You can take a crop and still leave behind extra for the soil.