And using custom fields, there's really no limit to what you can do. They won't be integrated as the standard [Seen], [Seen date] and filter are, but that probably wouldn't be practical anyway. Consider this...
Use the stock [Seen], [Seen date], [Rating] and [Comment] fields for recording the last viewing, regardless of who did the viewing. But you have a set of custom fields (the same one's) for each family member. You're probably joking about the cat, but you might want to include another set for guests (using the comment field to record who they were). The kids may not be interested in rating what they see, but willing to 'like' a movie. If so, give them a boolean [Like] instead of a [Rating]. Since some movies will be watched more than once, include a [Seen previously] boolean and record any available details (i.e., date, rating, with whom) in the person's [Comment].
This may sound like a nightmare to maintain, but it doesn't need to be. All of these fields would be presented in a separate section or tab of the skin as a table. The standard fields are going to be used in the usual way. The primary role of the custom fields will be to answer your question: Who saw the movie on [Seen date]? That doesn't get easier than ticking the [Seen] checkboxes for those people. A person's [Seen date] and [Rating] would only be used if that information differs from that recorded in the standard fields. [Seen date] could be left at a previously recorded value for a person, while the corresponding [Seen] flag is used to indicate if they attended the current viewing. All of the user fields could be set to be hidden if empty.
To get things started, I imagine you would just do the best you can to select the movies each person has seen, and set their [Seen] using the mass editor. If you happen to have some other kind of historical record (e.g., dates movies were seen or acquired), that could be imported. You might find it easier to add another field to record where a movie came from—you, your wife, a joint purchase, gift, etc. All this would require some work (mainly in creating or customizing a skin to accommodate the custom fields), but the result will be exactly what you need, and therefore much better than any additional feature could provide.