What I did once was to find a thin steel washer that fit over the derailleur bolt. Then I clamped just a small edge in a shop vise and hammered it over until I had just a small lip bent over. Then I installed a modernish shimano derailleur, using the washer lip to bridge the gap between where the Huret tab is and where derailleur wanted it to be. It worked well enough to run 8spd STI levers on the bike with probably 28t cassette.

I'm pretty sure I still have that washer, I'll see if I can find it and take pics of it in action.