Mission accomplished, thanks to a combination of methods offered here and other forums:
The new pulley bolt arrived today. I removed the bottom pulley and with the chain on the smallest sprocket, I was able to rotate the derailleur's outer cage plate after having removed the limiter-screw. This allowed the bolt to turn anti-clockwise enough so that, when I inserted the allen-key and put downward pressure on the tool, I managed to get sufficient bite into the bolt to get it fully turned and removed. Hallelujah!
Thank goodness I didn't have to resort to drilling etc. Thanks also to the denizens of this thread for the inspiration, freely given, which spawned a solution to a tiny bolt with a big headache.
Last edited by Gummomarx; 06-24-22 at 05:19 PM.