I saw a picture today of a combine with an air seeder on the back of it.  I didn't read anything more but it got me thinking.  Combines have a large fan and spread residue across the width of the header if set correctly.  Is there any reason a gandy box couldn't be retrofitted to the rear of the machine so that it deposits the cover crop seed into the spreader and distributes it with the crop residue?  I'm thinking radishes, turnips, and cereal rye while harvesting soybeans in mid/late September in Central Nebraska.