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I don't know if you are aware of this, but there is already a similar feature in OmniGraffle. When you select a shape, all its properties are displayed in the status bar at the bottom of the document window (the row of symbols to the right of the cogwheel). You can then select one of the properties (fill, line, etc.), or all the properties at once (leftmost button), and simply drag it onto another shape to apply the same formatting.

Hope this helps!