Quilt machine CNC conversion suggestions?

Howdy,

I have a small quilting machine to which I'm considering adding 2D CNC capability.
The precision required is pretty low compaired to most CNC systems, so I thought
it might be a good project for my first attempt at building a CNC system.

The system consists of a largish straight-stich sewing machine weighing about 25 or so pounds, mounted on a free-motion x-y table composed of pieces of aluminum 'L' channel, with about 15 inches of Y travel and an option of 66, 85 or 125 inches of Y travel (base frame is 66", with two extensions of different lengths). The frame has a couple of rollers and tension straps to hold the material to be quilted, so its well controlled and will not become entangled in any mechanical bits (I'm not planning on including advancing the fabric under the CNC control, at least, not anytime soon). Maximum linear speed is 120 inches per minute.

Probably the hardest part is that the X direction is so large. I'm not sure what options I should investigate for controlling this axis. The most obvious would be a long threaded rod driven by a stepper. Unless I can find some very cheap machined threaded rod in such large lengths (which would likely be very awkward to swap out and store, particularly for the operator of this particular machine :) ), I'd be stuck using industrial threaded rod. It also makes free-motion use more difficult.

A better option would to put a servo motor on the carrage and drive the wheels, with an optical encoder reading marks in the track to form a closed-loop servo system. I'm not familiar with the options for driving such a beast though. I could hack a controller together, but I'd rather use a standard controller so I'm not reinventing things. What kind of feedback do CNC servo controllers usually want? This option also allows the machine to continue to be used as a free-motion platform, which is a definite advantage.

I will have the Taig CNC mill available for making parts for this project (its one of my excus... um.. reasons, for getting the Taig mill), so a few custom bits should be required :)

Suggestions or comments?
Thanks
CS