There were a few patents for such things in the mid 1890s and post 1900. I seem to remember that they were for guitars and mandolins primarily while citing banjos to cover all grounds.
One patent that was banjo specific had individual bridges for each string.
That said, in the countless piles of banjo related publications I don't recall ever seeing one advertised.
I have not found that they are necessary for gut or nylon strings. The designs on the market today are more suited to wire strings as they are way over built for nylon.
Thanks Joel! I figured it was about the end of the 19th century. Now, I suspect Joel already knows, but others may find it interesting that compensated and bridges and compenstated frets had been in existance since the 16th century at least, with instruments such as citterns, bandoras, and pandoras. Of course this has not a lot to do with banjos . . . but more than a few luthiers hired to make banjos in the shops of Boston and New York in the late 19th century came from Europe and were no doubt aware of such instruments. Chris - individual bridges does sound nutty at first glance, but a luthier friend of mine tried it once. We predicted individual bridges would not resonate as well as a single bridge . . . but the individual bridges actually sounded better! However they were so small they kept falling over. I suppose the issue could have been solved but he never pursued the idea of individual bridges any further. However the idea absolutely has merit!