i'm not suggesting they buy index futures, but you can only roflmao so much. the idea has been suggested openly by past fed governors
in todays market, correlations are so tight and everything is such a monetary trade, it almost wouldn't matter what they trade, any major liquid market can move all the others. ie at the right time i think currencies or bonds, etc could be sufficient to stifle short term positions in other markets, commodities, etc
i don't even begrudge them the ability to intervene, so much as the possibility that done via banks it could be a for-profit advantage over the rest of the marketplace