Steve,
I think so much depends on what you have available in the way of wood working equipment. I am now thinking of a pull over circular saw on a guided arm so that you can cut across the sheet, with it clamped between two pieces of ply or chipboard. The sacrificial material could be confined to a broad strip covering the area in which the cut is to be made.
I think you would get away with a standard carbide tipped saw blade here as long as the cut isn't rushed. The paxolin will be prevented from chipping as the blade slices through the sandwich, but you need a big table or vertical sheet cutting facility to do a cut of that size; the rig would be like those you would find at the timber merchants where they do sheet cutting to size
Regards
Brian