Hi mate, great to hear you have the heads at last! I've seen those Crusader knights and they look really cool. You could easily create a backstory for a unit from Lebennin. Many were found fighting at Pelargir when the Grey Company arrived, the Dead Host frightening all but Angbor who stood firm, fearless unto the last...
Lebennin was the largest of the Fiefdoms and yet none travelled with Imrahil to Minas Tirith. I would have guessed they were busy protecting the villages, forts and ports along the river in preparation for the Corsairs or any Orc raiding parties that might pass through. When Aragorn sailed up the Anduin to the aid of Gondor he took with him men from Lebennin and Lamedon (probably well armed men from Lamedon, not the hillmen). He also sent 4000 marching north with Angbor who arrived a day or two later. This force probably recruited more men from Lebennin along the way).
I have only created a generic 'militia' for the men of Lebennin but it is highly likely there were multiple factions (some of whom could wear those helmets perhaps).
You could also use the helmets for Golasgils personal guard or for the entire force of Pinnath Gelin who wore proud green garments (over mail)
Good luck
