Hi David,
Off the to of my head without seeing the setup:
1. Place a ring spanner on top the draw bar and allow it to rotate against a hard stop with a soft packer so it doesn't chip the paint that comes off easily
then try to get hold of the boring tool with both hands and unscrew.
2. Does the boring tool have a scalloped bottom end which could fit in between the jaws of a vice, loosley, again with soft packing but not clamped? I'm guesing you have a vice of some kind on the milling table. Lower the tool in between the vice jaws just to stop it turning, as I say not tightened and undo the draw bar.
3. If the tool has the usual 3 holes in the bottom do you have a couple of pieces of scrap bar or stock that you could put in the outer most holes, then lower the head of the mill so that these pass between the vice jaws loosely and undo the draw bar,
Ed.