I have not needed to do this, and I will admit I've not tried to do it either, but I would use a workflow rule that runs whenever a new manager is added to the milestone. The workflow rule could then assign the same user as a manager to the parent project for the milestone.
As I say I've not tried to actually do this yet, so forgive me if this doesn't work properly. Hopefully it will help you move in the right direction though.
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