It does not appear to work: execution does not stop when the variable changes value

I must misunderstand some of what you wrote. The only times I see it not stopping (in your example output) is exactly those places where I thought you had requested that it not stop.

and the L command does not show the watchpoint

Yeah, you might have to debug the code. I don't think I had that problem when I ran it years ago, so I'm not planning on trying to reproduce your problem in order to debug for you at this point (especially since it seems that I am misunderstanding something).