A friend of mine recently bought a Ektelon DE with an H upgrade. This machine was previously owned by Wayne Zeyen, a well known stringer in the Stockton area. He passed away a while back and his family was selling his machine.
The machine had a half glide bar clamp. I don't know if this was a standard part.
On a rare occasion, I have to agree with Irvin. An ATW can be done with a glide bar machine, it's just more difficult. Basically when you start the ATW, you will need to move the glide bar perpendicularly (90 degrees), while holding the string in the tensioner. Though it can be done without a starting clamp, though it would be helpful. (this is where I disagree with Irvin) If you make a mistake, you are SOL. Which brings me back to the half bar clamp. I think it can be used to clamp the perpendicular string while the full glide bar is still holding the previous string. I'll have to give it a try, to make sure, but I think it's possible.
Machines: Gamma 6004 2-point w/ Wise 2086 & Babolat Sensor Dual