I did a little more playing with the power window motor this evening...
It seems to have plenty of power for using it as a x-axis drive. Still a little slower than I would be happy with at 12v. If you crank the voltage up, the RPMs increase as well, and at 20v it moved the table at a reasonable clip. No clue how long it would last being used that way though. Since it doesn't seem to be struggling I suspect it could be run at 12v and geared up and still have plenty of torque to operate on that axis.
I then decided to give it another shot at the z-axis handle. Pushing the drive voltage to 16v and the current to about 6 amps, the motor actually does have enough torque to both raise and lower the head, but just barely. Also, the motor heats up pretty quickly, not terribly hot, but hot enough that I wouldn't trust it to last very long. And even over driving it like that, the speed isn't quite acceptable.
I've got a 1/2HP old garage door opening that I'm certain would be plenty powerful enough. I've seen where at least one guy managed to hook one up to drive his z-axis and it seemed very successful. His mill was larger than the G0704 though and had a reasonable place on top of the column to mount it. I suspect it would be very out-of-place on the top of this mill's column...
I'm considering trying a wiper motor if I can source one cheaply enough (read: free). I've read that some don't like running in reverse, so that could be a concern with that approach...