I'm wondering if a corduroy casual jacket, perhaps shortened if need be, would be considered 'over-dressed' for your tastes? I'm not talking about a corduroy sport coat, but more like a regular light jacket.

And what about just a vest? If the temperatures you're talking about are too warm for a tweed jacket but too cool to go without something, maybe a vest of some sort would be just the ticket? Not even a proper "waistcoat", but maybe a duck/canvas or denim vest? Anything without sleeves can be used to give a little warmth in your core/torso while presenting a "dressed down" look.