Yeah, thanks! I looked up the ECO S on the www, and it wraps 31 teeth max - not enough. The Duopar is a bit bent, sooooo, since I already have Suntour barends mounted, I'm going to try a Suntour on the rear. I've got these:

7-GT

XC Sport 7000

Accushift -4050

The biggest factor against them is that Suntour rear derailleurs are not at all compatible with Simplex dropout mounts. The PX-10 came with Suntour when I bought it second-hand, and it made wheel changes a challenge. I might try some sort of quick-release skewer...