WAI,
From what I understand, the darkening process is due to oxidation. I have heard of folks using chemicals such as oven cleaner or potassium permanganate (I think thats spelled right) to quickly oxidize the surface of the bow. You could do your tape thing then hit it with the spray on oven cleaner and let it sit for a bit. Wipe/clean off and there ya have it. A coat of spray finish with UV inhibitors wouold prolong the effect. Even with out the spray finish, it would darken naturally with the pattern still there.
Give it a try like Pat said, and be sure to let us see how it turns out.
OkKeith