How can I create a photo slide show of one face from iPhoto's faces?
First: You can't do it with iPhoto, it's just not a feature.

Next: Those faces thumbnails are stored inside the iPhoto Library package - in the Data folder on some versions, the Thumbnails folder on later ones.

If you make an alias to that folder you could possibly access them that way using another app. However, they are scattered hither and yon throughout the folder, in multiple subfolders and may well be difficult to find. That said they include they all include _face0 in the filename so a Smart Folder will help find them

But... However, these are very small (200 - 300 pixel on the longest side) images and it's unlikely that they will perform well in a slideshow.