Personally, I would not use aluminum.....to me that basically equates with "cheap"....not to mention the soft characteristics of aluminum would mean things would get dinged up quickly. If a person wanted to do it, there are aluminum rods. Using any "rod" material other then one that matches the bolster composition would mean they would certainly show.
Ruana knives....which are made here in Montana have cast-on aluminum guards and/or bolsters......and everyone that I've ever seen looks terrible to me.....most are poorly finished, but even on those that have a decent finish, they are discolored and dinged whenever I've seen them.
If a person "wants" to use aluminum, they certainly could, but in my mind the question would be "Why would you want to?" By that I mean why subject yourself to the negative implications of using it on your reputation.