Geezer here: I've seen renaissance crossbows that probably drew @1000 lb. at @ 6 inches, that were fitted with bone/antler nut. The stronger ones usually had steel reinforcing pins thru the lugs and down into the body of the roller, and at least on the big siege bow at Vienna (Kunsthistorisches Museum) the roller is quite large... perhaps 2 inches in diameter and wide. But I think really good antler should be up to the task. I've also seen some very powerful Flemish bows (there's one in the French Army Museum in Paris) that used solid brass or bronze roller-nut. Again probably with a steel sear-block. In fact, 1000 lb. isn't so very strong for later Renaissance crossbows, and in fact, the power produced isn't awfully great. Probably no more than a strong police pistol... say 9 mm or .45 auto.
As far as cast iron rollers... yeah, I think I would stay clear of that for a strong bow, not because iron isn't up to the task, but cast iron of that sort was never intended for the job. There may be lots of slag inclusions and the stuff can be brittle.
And not to be a noodge, but there's a reason medieval bowyers used the materials they did. If you wanna make bows like theirs, maybe you oughta take a long look at their preferred materials. If you wanna make modern bows, take a good look at what's available on modern material specs. You should be able to make a lock that's plenty strong... but you may NOT be able to make a good one out of compressed dog-poop, reinforced with wallaby hairs or some other found materials from grandpa's garage.
Nuff sed. Geezer.