Another update:
Yesterday afternoon, I was able to find a Ford parts retailer that was kind enough to pull a mirror from a F-150 and an Expedition off his shelf. We compared the mirrors' mounting "points" and realized the mirrors are WAY different!
It would take some serious fabrication and modification to use the F-150 mirrors on an Expedition.
Hopefully for 2012, Ford will offer a towing mirror option for the Expedition.
Until then I will be using Reese towing mirrors attached to the factory Expedition mirrors when pulling my trailer.
Marc