Geezer here, trying to answer cogently (suffering from the Mother of all Allergy Attacks-imagine this note punctuated with drool)
As for 'period' bows with steel/iron nuts, it depends on how you look at it. I've seen a bow in the French Army Museum in Paris (as I recall) with a bronze nut... the caption claimed it was medieval... one of those heavy Flemish deals with the square console on bottom.... in fact it might have been 17th century. The Metropolitan has a lovely little german-style self-spanning bow that's a combination stonebow/boltbow/wheel-lock. That one has a shiny steel lock and steel bolt-track. It might be as late as 17th century. Certainly some of the really big wall-bows or heaviest field-bows might use metal rollers, but in fact inertia of a large metal nut does seem to be an important factor in misfires... the damned thing just wont accelerate fast enough and the string climbs over the lugs on release.
As for aluminum, I think I've mentioned this before, but here goes. First: aluminum is quite unpleasant to work.... it's sort-of sticky- fouls up your tools and builds up lots of heat when you work it. But if you persevere, you'll eventually get a beautiful, shiny lightweight roller. You'll want to insert some sort of hard metal sear for the trigger to work on, and that may lead to weird bi-metallic interactions later, but let us forge on. Eventually you make that beautiful aluminum roller, place it in the socket and get to work shooting. You'll discover the roller oxidizes around the periphery pretty quickly. Over the next few months, you'll get a buildup of grey oxide on the inside of the socket. If the bow is left unused for a few weeks, the roller will tend to stick to the oxide-deposit in the socket. So your first few shots will tend to hang up, possibly causing misfire, or just longer lock time and be a pest when resetting for follow-up shots. It's mostly just a nuisance, not a game-changer. But then you discover a grey oxide buildup on your bowstring, which is being distributed along the track with every shot, and honestly, it just looks nasty. Again, it isn't necessarily a project killer, but the beautiful piece with its shiny/silvery lock starts to look like something from a run-down tenement. After making 3 or 4 aluminum rollers, I decided it was no more trouble to turn a piece of moose-horn stem and do it right the first time. So I'd say, if you want to do medieval locks, there are three ways to go: First, build the roller in hardwood, with a solid steel plug sear. This will work great on lightweight practice bows (100 lb. draw or less) Second choice, go for an industrial plastic like Delrin... they can be got in white or black, the stuff works great and doesn't look too tacky. Good for bows of a 200 lb. or so. Third: invest in a chunk of real moose or axis-stag antler big enough to lathe-turn down to a cylinder and use that. Most of the other alternatives I have seen will produce an inferior product even after hours of careful work and thought. Those ancient bowyers knew their business. If you're gonna make it the way they did, you probably oughta pay attention to their methods. Geezer