When I had my Della Santa built 2 years ago, Roland showed me a tool that Mario Confente built to address this. It was a 14 inch long lever (made from a chain stay) that had 2 pegs brazed on at one end. One peg went in the derailer mount, the other went in the drop out. You then could pivot the drop out useing the lever. The tool was very nicely finished(no lugs or cutouts;-)) and even had a little slot cut on the back to hang it on the wall.
Do not worry about riding with a drop out that has been straightened. All of
them have been to one degree or another....
Garry Nold
Kent, WA