The chains were too close to the blade when you backed up? We wondered about that. It sure looked nice, though. The brush hog stops for nothing. I think a rubber flap will get eaten, too. You would have to make some sort of extension to keep the flap or chains back far enough so that they cannot touch the blades if folded under while backing up.
I was out brush hogging with the PT425 Monday night and it sure is a beast. Stuff flies everywhere. A leaf blower does a good job of cleaning things up off the front of the tractor afterwards. I am going to invest in a full face guard and hard hat, though. I was getting slapped by multiflora rose branches after they flip past the ROPS... OUCH!!!