If you go to the Modern Game Bantam Club of America website, look under Library... there are many articles about Moderns. In particular, look for the Feather Conditioning article. It talks about using cod liver oil in their diet, as well as on the feathers. I would be very careful about overdosing with any vitamin supplement, but used sparingly, it makes sense.
What I do is this... use anti-frizz hair serum on the feathers. I only do this for shows, because I prefer to use an actual conditioner on a weekly basis. Spray a little on the feather itself and "pet" it in... go with the feather, never rub backward. I use an old silk scarf to work it in. I have never washed the silk, and it seems to keep getting better the more I use it! It gives it an amazing shine and seems to realign the barbs in the feathers. Give it a try and see what it does. It is the same anti-frizz serum I use in my hair... you can find it at the drug store. I prefer to use Citre Shine, but any brand will work. The serum also repels dust if used sparingly.