I made a convoluted "fix: to do that for the first year of my ownership of a Raleigh Competition. Forget what I did. Worked well but making and installing it wasn't easy. That bike then went to be stripped, inspected and repainted as I didn't trust the workmanship. While there. the dropout was modified to the new standard. (I assume with a piece of steel brazed in. Bike also got the seat lug pin area completely overhauled to the current seatpin standard, WB bosses and braze for the lugs! Enough work that I never asked about the hanger stop details.)

Thinking about that washer just now - I wonder if you could shape a little piece of say 1/4" plate 6061 aluminum to a "parallelogram" with arcs instead of circles top and bottom to (looking from the side) fit the hanger and stop with the back flat of the shape serving as the new stop. Drill a countersunk hole in a large fender washer and a matching tapped hole in the new small piece. (It would take a tiny screw to do this but it only needs to keep it from falling off.) Ace Hardware probably has the perfect flat head metric SS screw for the job.

I'd get a stainless steel fender washer with a smaller inside diameter than the hanger bolt and drill to just clear the bolt. Might well have to cut down the rest of the bolt to clear derailleur, chain, cog, etc, leaving just the tab for the new piece.