I was just thinking about this in the Paladin thread over in General Discussion...
Ride-by-Attack, Trample and other maneuvers should be expanded uses of the Ride skill, accessed through the "Mounted Combat" feat. You choose to be trained in Ride, take the Mounted Combat feat, and you gain all those nifty options while riding (like the feats in PHBII that opened up new uses for Balance, Diplomacy and whatnot). If needed, scale up the abilities gained (like saying "At 9th level, you deal triple damage on a mounted charge (quadruple with a lance)").