I ended up purchasing IAH-LHR // LHR-MXP // MXP-LHR-LAS-PHX-IAH. I'll jump off in Las Vegas once I hit home. Cost per passenger was $2,179.33. I used ITA Matrix to pull up the itinerary, BookWithMatrix to generate a bookable link, and then went through the American Airlines site to actually book it (and was able to use a bunch of gift cards from various American Airlines credits).
Pretty stoked for a couple of days in Milan at the end of a trip I was already taking.