I've had the same challenge on almost every quill stem. With the exception of the ITM Eclipse which has small radius cuts on the backside of the clamping area. That little cutout made a huge difference when I would slide the bar in. When I encountered a curve in the bar, I could rotate the bar so that the inside of the curve would be at the cutout making installation smooth and scratch free.

I wonder if temporarily lubricating the bar in the same fashion that is used to install grips on mountain bikes. Soapy water on the bar side that will be slipped thru the quill. The soap will evaporate leaving no slippery substances that could affect your installation later.

Or do they make quill stems that have a removable endcap?