it might be posible. I always liked looking at older engines from late 1800's as well as casrs & trucks you would be suprised to see so much as fabricated rods that do several things at once & some custom fabricated that were bent & trail fitted on the machine or repaired by blacksmiths.
if your bike is sprung you would need to come back to the seat post area & make sure brace is triangulated preferable below the seat post as to not stress the area above the frame & then you will need to do some figuring as to balance of a rear passenger & you might have to move the banana seat forward & then locating where to place rear pegs & how ackward handlebars may be while doubling & have to plan on different bars or maybe at least turned different
Later,
Randy