... for short positions and thus do not qualify for the stock yield enhancement program? Is there anywhere you can check whether a specific long position is being used as collateral? Alternatively some aggregate measure that indicate whether the margin requirement of the short positions means long positions will be used as collateral?
you can check each securities margin requirement and it is stated clearly by ibkr. if a stock margin requirement is 100%, that means the stock cannot be margined. US stock is usually more marginable (the percentage of margin requirement is a lot smaller) than European or Asian stocks.