I can't confirm it 100%, because I have all of the villagers except some of the new ones, but I tend to doubt they would keep you from obtaining the item just because you can't host, say, Filbert. They would probably lock the Filbert cookies if that was the case.
Ya, I didn't want to answer because I wasn't 100% sure either ( already have all campers required for all memories up to now ) but I don't think they would lock up an item until you get the campers invited or hosted. Especially if their goal is to make money. I don't think they would place a restriction through a free feature ( hosting / inviting campers ) before being able to spend money to achieve something ( getting the item required for the memory ).
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