I think I've got the problem solved. I pulled a long-forgotten Sachs/Huret long cage derailer out of a drawer and installed it. Now I just have to get to the LBS and buy one of their overpriced ss shift cables.

I had it on an old Peugeot mountain bike for a while and it worked great with Shimano 7 speed shifters. I call it my mystery derailer because I've never seen one like it and no one I've shown it to has either. The lable says Neos. I got it from Europe, so maybe they were never sold in the US.

Just look at how close the slant parallelogram puts the jockey pulley to the cogs.