Here's a really helpful thread about freezing mousse. The best recommendation is to making the mousse through the step where you add cream, and then freeze that puree. When you want to serve it, let the puree thaw fully, put it back in the food processor, add the cream and capers, and you'll be good to go!
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http://www.food52.com/hotline/4511-can-you-freeze-chicken-liver-mousse-pate