On placing an order, the fiat and BTC (or other cryptocurrency) goes into the network and is unavailable to the trader.
If we could do that, we wouldn't need Bitcoin
As for mining fiat, the problem of getting USD into mined fiat is exactly the same as getting USD into Bitcin, and at that point you might as well just transfer it into Bitcoin from the start.
As for many small escrows, I'm not sure how that is different from the single randomly chosen Dwolla escrow I proposed.