You could always hard coat anodize the shaft. If you make a larger quantity, and plan on selling them, the anodizers charger per the batch, and you should be able to make out OK with the anodizing cost. The anodizing usually adds around .001 inch to the size of the part, so you need to be careful of this. You will also need to be careful what grade of aluminum you are using, because some aluminums do nod anodize very well. We use a company called Wepco located in Massachusetts at work for all of our anodizing. You could also set up to anodize your own parts. I understand that it is a pretty simple process.
With the plastics you would need to look at the lubrication properties. If it has a high lubricity, than it wouldn't wear the shaft as fast. Another choice is to install a steel sleeve on the aluminum shaft at the wear areas. Someone had a post on doing this recently. You need to be careful putting steel against aluminum since the dissimilar metals will react with each other causing oxidation. I work with these materials a lot at my job. I fabricate machinery for a medical product manufacturer.
