Thanks to everyone who answered my plea for a really thin cable. Seth F. measured a real Juy 543 tension cable for me, and it's 0.75 mm in diameter. That's probably a lot smaller than old French derailleur cables. Fortunately, smallparts.com has 0.8 mm stainless cable. It's $3 for 10 feet which should be more than enough cable for a lifetime of 543's unless you're Kauzo.
Leonard Bulger
Marblehead, MA