That is a tough one. I am not sure what I would INTERPLANT with existing plants. I guess I would plant something that definitely dies and stays short. Peanuts? If you fail, then it all gets dug in anyway. Soybeans? Too tall? The great thing about beans is that they are cheap, a legume, and edible. Yum. Consider sweet potatoes. They crawl. Pumpkins are unruly, but they will not propagate and are easily killed. Each of these is mildly ornamental. Or just mulch it with something that will degrade slowly over the course of months or a year.

I would agree with borderbarb that there's probably not enough surface area to grow a lot of green manure but it's sounds like something I might try for the heck of it. Right now I have sunflowers and sweet peas growing out of compost covered hay bales. To what end I'm not entirely sure but they should look cool if they take off. I think if you want to grow your greens for your pile you're probably better off setting aside a section of your garden for it. Comfrey is supposed to be excellent for that but it is invasive. I have enough invasive plants growing already so I've kept away from comfrey. I'm growing some borage this year and that is supposedly a good green for the compost pile. Are you sure you can't scrounge up some greens? A quick search of Cleveland's craiglist showed a handful of people getting rid of hay which is usually a pretty good green in the compost pile. It helps having a pickup truck to get those sort of things.

While Oats are generally a spring crop you can still seed and grow them now as a quick cover crop. However, if you want a winter cover crop something like Field, Cereal, or Winter Rye (all the same) would be better.

I have not noticed a problem with Buckwheat being suppressd by Juglone, but have not used Oats at all. Buckwheat has allelopathic properties of its own.