My experience parallels Brad's and Fiver's comments, but another possibility, without going to the trouble of making a special alloy, is a metal which is highly popular in knife materials and made to do what you desire - nickle-silver. Your knife-maker is likely very aware of this material and may have already had a reason to use pewter instead. I don't think nickle-silver melts nearly as easily as pewter and he may have it in his heart to incorporate that method skill for the sake of his art or historical purposes. I believe nickle silver only became really popular as a knife bolster material in the Western World a hundred years ago or so, after the age of most being made one-off.
In my personal experience, nickle-silver seems to tarnish much less easily than pewter and in working it, it seems as hard or harder than most common brass I've used which has not been annealed. It's not terribly expensive either, only slightly more than brass. I have sulfur in my well water - not enough to smell after going through a plain old softener, but my wife has some pewter jewelry which will tarnish in open air quickly while the nickle-silver on my knives is not at all affected. It works almost like brass but "feels" a little harder when filing. Rods, tubes, bars and blocks are readily available as well. My opinion on this is prejudiced toward nickle silver because I also like look the look of it and it makes a great front sight on a muzzle-loader. Your preferences and your knife-maker's experience may differ for whatever reason but it's a material to look at.