I don't see a problem with a duster, since it is a type of overcoat. Thompson recommends that one wear a trenchcoat with a kilt if you do not have an Inverness Cape, a traditional outer garment for a kilt.

I wear my "barn coat" from Woolrich (L.L. Bean makes a nice one as well) on occasion with my kilt, and my best friend wears his Barbour wax jacket with his. Very much the country gentleman look.

Cheers,

T.