There may or may not be poison pills to stop MIG shareholders...

There may or may not be poison pills to stop MIG shareholders getting rid of Macquarie, but who are MBL's major shareholders? The same as those of MIG. Capital Group Companies and the Commonwealth bank own almost 14% between the two. Actually getting more than 50% ownership by one fund or group will be difficult.

To quote the Australian: Ref http://www.finance.news.com.au/story/0,10166,18408912-14334,00.html

Investors and traders contacted by The Australian hosed down speculation that Macquarie would be ousted any time soon. But most believed that offshore hedge funds were buying the stock on the basis that Macquarie could be ousted and fees cut by more than half.

and...

Although Macquarie can be removed as manager of MIG with a 50.1 per cent vote, many of the institutions that own MIG also own Macquarie. Colonial First State and Capital Research & Management Company each own significant stakes in both companies. They seem to have no interest in undermining the Macquarie model.

At the end of the day MBL could just give MIG a reduction or discount on fees and the speculation would end right there. I wouldn't be betting on MBL getting ousted.