My concern with using a piano hinge on this is the armour is abs plastic. Not solid resin or fiberglass like what most Fett gauntlets are. Also, Ash does have rather small forearms so a slide one would be better suited.
As far as padding goes, I had I similar problem padding the forearms on my tk with padding coming out but I fixed this by using thinner PVC pipe insulator cut lengthways wrapped around and using a platiprime glue which is designed specificly for rubber and foam.