Dear Mark,
Firstly, I would place a drop of T-9 at the junction of each nipple/spoke. T-9 does a remarkably good job at penetrating and freeing-up old corroded threads.
Secondly, just use a pair of slip-joint pliers to grip the spoke while you loosen the nipple with your choice of nipple-key. This is often best accomplished in the truing stand. Break as many free as possible and cut out the remaining uncooperative spokes.
Voila'
Mike Fabian
San Francisco