I am using a Bitcoin plugin written by jalder. Both the jalder and bitpay plugin would leave me having to update the prices in the store from time to time.
So if you could enter the price in dollars but it would always calculated the equivalent price in BTC based on the 24 hour VWAP from MTGox BTC/USD, that would be all you need?
Looks like you are using round numbers, so does simply having a display round down at a certain number of decimals work?
e.g., for something priced at $25 USD would display 1.8 BTC which is what 1.85185 BTC is rounded down to, using $13.50 BTC/USD as an example.