So you really want a
perennial cover crop? That would be a crop that doesn't ever die off. How would you plan on planting your annual crops in the midst of the perennial cover?
Going down the lots of mulch route is better. If you need more organic material in your soil, you can do a single annual cover crop, and when that is done, pile on 3 to 6 inches of mulch on top. Use the chop and drop method, keep adding mulch, and you
should not need any more cover crops ever.