http://www.sonic-youth.com/mustang has a bunch of pictures and info, I'd recommend checking it out.
Generally, Thurston prefers using Seymour-Duncan replacement pickups (like the SJM-3), while Lee prefers humbuckers (like the Tele Deluxe pickup). They used to be rather fond of tossing TOMs on the guitars, but more recently acquired guitars generally look like they've been fitted with stang bridges. Usually they rip out the rhythm circuit, and besides that it's pedals, alternate tunings, and the behind-the-bridge chimey sounds that's doing the work. Kim's been playing guitar a lot, too, ever since Washing Machine, though she generally seems to prefer "simpler" guitars (like the Les Paul Jr.).