you can put one of those car bra's on the front, it will protect the paint from rock chips, make sure to applie several coats of wax under the bra or it will chaif the paint over time. personally the bra look silly to me, but they would be effective for what your looking for.
only other thing I can think of is instead of mud flaps, they make those brush thingy's that hang off the back of coaches and keeps rocks from kicking up.