Brian
I have another option although I am yet to test it but I have a lathe I am retrofitting at the moment. It has very large DC Servos on it and I had hoped that the CNC Teknix Tek10 drives would suffice but alas they didnt. Anyway I knew the Tek10 boards could put out an analogue signal but as my lower (power boards) had blown I asked Pete at Teknix if there was any way to use just the top boards, seems if I jumper across two pins I can trick the top board into thinking the lower board is there and working. So in theory I can use the top boards to convert the step/dir to analogue, as I said I havent tested it out yet.
I am not sure how much it would be for the CPU board on its own, or even if Teknix would sell the CPU board on its own. Might be worth an email to them though and one thing is for sure, you WILL get a reply (remember though they are in Aus so the time difference might cause a delay)
Hood