Is your friend looking for wiring diagrams to do this himself or just to know if it possible? It can definitely be done, and you could split the Crawler using a push pull pot then just take the hot lead from that and run it to the switch and wire the guitar as normal. As Dave Sloven suggested, you can also use an on-on mini toggle (which is what I did for series, parallel, split for the Holy Diver I wired up). I can probably dig up and post a wiring diagram if your friend wants.