I think the main difference between your idea and mine is the fact that yours tries to smooth out and control the volatility of the market, whereas mine thrives on it. Merchants should see their investment in bitcoin for exactly what it is; a gamble. Right now, I'd say it's a good gamble, as prices have risen astronomically since it's inception. This can be used as a selling point for more merchants to try it. I think tying it into a peerfinding system like (but not limited to) MtGox to allow them to pull their money out instantly is crucial.
I'm really interested in the proliferation of this kind of technology, and I don't think I'm alone when I say the future viability of bitcoin may depend on these types of systems being in place.
In the end I think most merchants don't want bitcoins. They want their USD or other fiats. Bitcoin is just a way of payment they want to use, not a goal on itself. If prices for bitcoin rise a lot, they can also fall a lot. This would mean they would have to sell at the same time they get the bitcoins, which might not be possible at that time or the price has dropped within the time of start from purchase by the customer and having the bitcoins sold again on the market by the merchant. For expensive sales this could cause quite a loss for the merchant.