Originally Posted by elcraft

Since you really only need a chain tensioner, rather than a derailleur, you could go really minimalist/era "apparently" appropriate and locate a Huret Luxe or Svelto derailleur. Then , using the very long limiting screws from an Huret Allvit, you could effectively "pin" the Derailleur at the ideal spot for the chainline. These steel derailleurs are lighter than an Allvit, but look plausible as OEM, and are more than adequate as a chain tensioner w/ their multiple position pulley cage tension.
Well, that's at least what I might try if I were in your unique and fortunate situation. I did something similar w/ limiting the travel of a Huret Jubilee for use with a triple cog derailleur set up. The Allvit limiting screws are fairly easy to acquire at a Co-Op or on eBay (you can salvage them from rusty or scarred items that wouldn't command a very high price.).