Just some thoughts, maybe someone did such a thing. Bess like arms, officers fusils were manufactured by private companies for private purchase. And the UK sent tons of besses to arm the Mexican army, some of these got in to private use as we know a band of outlaws was caught by the Texas rangers and disarmed. They had old besses among their arms
But I would hazzard if it was done it would be very rare. The stock profile fell out of favor and guns faced a lot of change.
A rib would have to be custom made and put on. This back in the day would have been hired gunmakes work. A farmer or frontiesman wouldn’t have had the tools necessary
A cracked forestock might be removed then pipes could be soldered directly to the barrel, leaving a pipe off and letting the rod bend a little
Of note was some New England guns that had wooden underribs, and such a thing could be made with minimal tools.
If we can’t point to an historic gun know that had such a modification that’s meaningless in of itself. A guy with a busted fore arm could have modified his gun to be workable. It would be a one off, and the back story to it isn’t off the wall. I cracked my forestock and fixed it thus, ain’t to hard to swallow
