Clive,
If you want to put fert down with JD put a front tank on the tractor and blow the fert back to a cyclone, this drops the fert into the bottom of the seed meter and is then mixed with the seed down the row. This is the way it as done with accord drills back in the day and the are probably quite a few still like that now. Someone's bound to have a picture.
AFAIK the JD has an accord type metering system so it is just a case of ducting the fert into the seed stream before the Venturi.
My worries about doing it this is if the airflow is enough to carry seed and fert at reasonable rates ? Also concerns over seedling scotch if fert and seed are not separated but I'm not sure if this is a real issue or just a myth ?