Just had an idea: "What about adding a movement limiter onto a flexible nib, to prevent overbending?"
There is already something like that. The Pilot Justus 95 Fountain Pen has a user adjustable overlay that not only affects the flexibility of the nib, it limits how much the nib can flex. Have a look at the pen here:
By the way, ignore the price on of the Pilot Justus 95 pen at the link above. That seller is in the U.S. where Pilot fountain pens cost about double the price you can find if you buy direct from a reseller in Japan.