Against all odds (in my unprofessional opinion), Dad's van made it back home safe and sound sans idler pulley. The remains of the plastic around the bearing was mostly smooth, and the tensioner obviously held it out far enough to keep just enough tension on the belt to keep the alternator spinning. He refrained from using the air conditioner as a matter of longevity (not like we needed that after leaving Bourke anyway, we're far enough south that we're actually coming into winter).
We'll assume the serpentine belt is no good now, as I'm sure that ~615mi trip will have dramatically shortened it's lifespan. Now we're looking for a cheap place to pick up an idler pulley without buying the entire tensioner - eBay looks promising.