Hey man, it would be awesome if we could have a favoriting system. It would be exceptionally easy to do as well, just add a new table to the database named Favorites, with columns for user ID and card ID. You could add to each card's page the number of times it has been favorited as well, with a simple aggregate function counting the number of times that a card's ID appears in the Favorites table.
Also I would love to help with the site's development, it's just that I'm swamped with school right now. If you're interested I can check in again once my load is a little lighter.
Backend-wise, it's certainly very simple, yes. Presentation-wise is where the effort would be: what does the favourite icon look like when it's checked, when it's not checked, writing the view to show all your currently favourited cards (and finding somewhere to link it from, especially as I don't really have a sensible user profile page at the moment).
But yeah, it's a good idea, and it wouldn't be too much effort.
As for helping out, many thanks for the offer; I am planning to put the source code up on Github so people can make their own changes and send them via pull requests. I've not had much time to work on Multiverse myself recently though.
I have an idea about presentation - put text below the card, under where the edit button goes now. When you build the page, check the DB for if it is favorited by the user currently logged in. Then if it is, write a string "Favorited" to the page. When you click that string a dialog box pops up giving you the option to unfavorite it. Vice versa for if it's not favorited.
You can give users the access to all their favorites through their user page (even if it is spartan now, you gotta start somewhere). It can be text based with hyperlinks to the card page, or potentially a visual spoiler.