If you know the index rules, it's fairly simple to calculate the turnover it is going to generate when it rebalances and subsequently the impact the rebalance is going to have on prices. This is a fairly well known behavior and a lot of players have been doing this for a long time. Basically every index of any significance is already being monitored and rebalancing effects are "front-run" this way.
I put the word in quotes, because this is a perfectly legitimate way of front running. The major indices are all public, and anyone can take a crack at this. In other words, if I announce to the world a month in advance that on a specific day and a very specific time of the day (at the closing auction) I'm going to buy X amount of specific stocks and sell Y amount in other stocks, I can't blame people for using that information against me.
I put the word in quotes, because this is a perfectly legitimate way of front running. The major indices are all public, and anyone can take a crack at this. In other words, if I announce to the world a month in advance that on a specific day and a very specific time of the day (at the closing auction) I'm going to buy X amount of specific stocks and sell Y amount in other stocks, I can't blame people for using that information against me.