I find that although bulky, I can't actually see any of it since it is hidden by the monitors... I suspect that having one monitor in portrait mode would make the mount rather more visible than in your case.
I was assuming your panel will be fixed to the wall by the runners. One at the top, one at the bottom... Ah, I see. No, my desk isn't hard against a wall, nor close enough to one for monitors mounted on it to be at a usuable distance. I would have to fix a backboard to the desk and mount the sliding panel on that: hence my comment about imagining the whole thing falling over forwards.
I'm inclining to your suggestion of the liner-rod guide idea; I have to find out what height the top of the bearers would be from the desk, and calculate the advantages or otherwise of routing out the shelf a little to accommodate them. My skills or otherwise with a router haven't be exercised in a good few years...
Bert