I doubt you will be able to plate this item in a conventional manner because the urethane is a very good insulator, and you don't have enough metal in the mix. The way you describe it, only 1/3 of the volume will be metallic. With this mix, the bronze particles will be insulated from one another fairly well.
You'll have to experiment with conductive paint or find someone to vacuum deposit a conductive surface such as aluminum on it for you. There's nothing "foolproof" out there. Some have had better luck than others with Silvaspray etc. It's always worth a try.
Ken
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