Sir Carl August von Alten commanded the 5th division at Waterloo. His division included 30th, 33rd, 69th, 73rd regiments.
He was first officer in Hanoverian army, then in the KGL.
In April 1812 Wellington, gave him command of the light division in Spain : 43rd, 52nd, 95th, 1st and 3rd Caçadores.
So yes an Hanoverian officer could command british officers and units.