The banks would NOT have to temporarily convert currency into bitcoin. I could see them sending blocks of fed notes (1,000,000's 100,000's 10,000's) with marked satoshi's/colored coins. Of course they would still have to pay transaction fees with bitcoins. I believe it would still be bullish from Bitcoin being acknowledged. It just would be a way more subdued trip to the moon since they would not need a billion dollars of bitcoin to transfer billion dollars of fed notes just a few marked satoshi's.
Can you please elaborate on how the block chain can be used to send fed notes (or other non BTC transactions) without using actual bitcoins?