You could always just use the search feature in the Memberlist add-on to act as a filtering engine. Memberlist v3.0 will actually make it possible to add search fields via the control panel (provided you have the profile field already installed, of course, but I'm going to make another hack for adding and removing extra fields via a CP as well), so you could set up a profile field for "favorite color" and then add it as a searchable field to Memberlist. To find all the users with "blue" as their favorite color, you'd just have to do a search for "blue" on that field.