New but little-noticed guidelines for American embassies approved by the Bush Administration could boost U.S. exporters. Under long-standing practice, U.S. diplomats and Commerce Dept. officials could not go to bat for an American company bidding on an overseas contract if rival bids from foreign suppliers included even minor U.S. participation. But under the new rules, Commerce can support whichever proposal is deemed to offer the biggest gains for the U.S. economy. Officials say the change will be particularly helpful in the Middle East and Latin America.