Hi Hollydog,
Yes, it is pretty easy to do it this way provided you can accept 'isolation routing' for the finished PCB.
Rather than explain the whole process here, this is a link to the way I have done it
http://www.cooperman.talktalk.net/files/15.htmIf you need to remove all, or just some of the unwanted copper then a program such as CopperCAM could produce the toolpath but this route produces a large GCode file with even larger machining times involved.
Tweakie.