One would expect that the code in Open Source projects is constantly inspected by the users of it

For my money, effective code review is done by the programmer(s) with other programmers. This way, you get the effect of new eyes saying "why did you do it this way?", to which the answer is either "because..." or "beats me..." (either is goodness :-) ).

has anyone tried to do a systematic code review on an OS project? Did it work? Do you think it can work?

If it's done to mark off an item on a check list, NO. If it's done to make the code better (see above), definitely (at least in my experience).

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