“Veritas makes a 6″ set of fence bars that should fit the plough plane also.”
I asked Lee Valley about this and they replied that they would not advise it. They said,
“Technically, the 6” fence rods are compatible with the small plow plane fence. In practice however, the rods on the small plow plane fence are fixed in place with a permanent adhesive that locks them in place. This is partly to ensure that the body, fence and rods work smoothly with each other. Trying to remove the original rods could damage the fence and make it unusable.”
That doesn’t mean it is impossible, and maybe they are being overly cautious, but that was their reply.