You can buy buckle guards.
Do you mean like those you use to protect the saddle flap from the girth buckle on a non-dressage girth? I never thought, you could use them lower down the billets on a dressage saddle and place them between horse and buckle. Not sure this would be a large/padded/smooth enough for my ridiculously sensitive and thin skinned horse, but it's certainly worth a thought.