Think the DigiSpeed is for spindle control only, dont think it will have close enough control of the voltage for an axis.
CNC Teknix sell Tek10 drives and the top part of the drive has analogue output from Step/Dir. I sort of tried it a while back, I put encoder pulses in and watched the voltage out and it certainly acted properly as if it was seeing the motor turning without Mach telling it to move and trying to correct it. I came upon step/dir drives that were suitable before I could get it all hooked up properly so didnt test any further.
I heard that Teknix were thinking of producing a converter board but dont know if it was true and if they have done so if it was. Might be worth emailing them, they may even sell the top board from the Tek10 drives seperately.
I have a couple of the top boards here (Scotland) and could loan you one to test if you pay for the shipping. I would need it back though as I may have a use for it in a future project.
Hood
Hood