Lifting throws are rather rare in armored circumstances and rely upon turning the opponent around your armor rather then taking their weight onto yourself in most circumstances. Within the art you have apparently been learning take a look at the Kukishinden Ryu Happo Biken when it is done by someone familiar with it. You'll notice nearly all of the throws require deeply stepping behind your opponent and turning them across your lower back and pelvis or very for the few that revolve around forward contact they are passed sharply off of your hips.
Traditional seoi nage - Grab the arm. Palm facing out with the arm braced against your shoulder. Break the arm (works on some armor, other armor will protect the arm) while lifting the opponent up. Place the opponents palm on the ground by sweeping your leg backward through their legs. (much like a roll)