The French general Chamorin wore dragoon uniform at campo Mayor - his brass helmet was taken by the 13th LD when he was killed. However, his was a recent promotion and he may have still been acting as colonel of the 26th Dragoons.
Lord Somerset wore a Lifeguard's helmet at Waterloo, but that was after his cocked hat was lost. Le Marchant apparently wore his pre-1812 light dragoon uniform and Tarleton helmet when he was a brigade commander in the Peninsula. I suspect that generals wore more of less what they chose to when in the field on active campaign.