Is there any way to feed the quadrature  output of the typical DRO scale into a BOB or similar interface so that it can be used as an absolute encoder for Mach? I would presume it would be possible since servos use an encoder.

You can feed an encoder or glass scale to Mach via the Parallel Port but Mach itself will not use that information.
You could use the information yourself by using macros, brains  etc but the trajectory planning will not use it.
One other thing is the parallel port is limited in the frequency that could reliably be input and 25KHz is likely the most you could expect.

You mention encoders and servos, with Mach it is the servo drive that normally closes the loop and as such Mach itself is still open loop. Some controllers used with Mach can themselves close the loop and update Machs DROs from the encoder position.
Hood

Looks like I need to study up on servos & drives in general. I'm wondering now whether I can feed a Gecko G320X wth the linear scales instead of a rotary encoder on the servo.

You would be able to use the glass scales with the Geckos but you would almost certainly end up with a very unstable machine. Few reasons but the main one is that the slightest amount of backlash in your system would lead to the motor constantly hunting back and forth.
Hood

What about the guy that built a very nice hydraulic press working under Mach3.

http://www.youtube.com/watch?v=38hfwvIbnjo

http://www.machsupport.com/forum/index.php/topic,13246.50.html

Quote from the guy that built the press brake

I got a little bit more done this weekend. I mounted the the linear scale on the ram, wired the motors, valve and started tuning the motors and valve. I still need to mount the home and limit switches. I got everything tuned pretty good. I can move the ram up and down several times and have it repeat within .0005. It is not real fast but it will get the job done. I even bent a piece of scrap I had laying around. I took a short video of the ram doing some 1" moves. I will post it up shortly. Hopefully next weekend I can totally finish this thing up up a bend a bunch of stuff. I will take a good video of the machine when I get it all running.

I don't know how he got the scales working with Mach3


Jeff