Fascinating! As a developer dealing with prosopagnosia, I've thought about stuff like this - especially given the advent of Google Glasses. This is something I'd love to have, but it would need to inconspicuous to make its use less intrusive than my taking guesses at who I am talking to and being wrong. So, I would want it to be in silent mode where I could inconspicuously respond to prompts and get the answers. Otherwise, I'll just continue to fake it in conversations with people till I logically deduce who they are.
Programming wise, I have debated whether or not it would be beneficial to use some of the coping methods that prosopagnosia suffers use when they are dealing with life. After all, many of us use logic trees for determining who we are looking at and speaking to. For example, we quite often will "recognize" people because locations most often have have a limited subset of people we know. Female, caucasian, short black haired individual at our church is always Chris. If I see her at a grocery store within 5 miles of the church, it probably still is Chris. So location can be a helpful way to subset the photos.
Location can be problematic as well, as many prosopagnosia suffers have the most difficulty when a person is not in their expected location. That is when we get it wrong. So your search must always include the entire pool even if the full pool search is a secondary search that can override your first result if a better match is found.
One instance where I expect Google would do better than me is two individual, female, heavier set, black medium length hair and play piano at our church on occasion. There are enough distinguishing features and shape differences in the faces that I know Google would be able to tell the differences, but I consistently have to work through it and my instincts usually get it wrong even though I know logically who that person most likely is based upon the current (not past) frequency of seeing them within 5 miles of the church and within the church itself. Even at other local churches, it is likely one of those two women, but at that point deciding which one is MUCH harder.
So there is a lot to think about here. Incorporating some of our behaviors might make searches faster, but it might also cause your engine to be prone to some of the same issues that make our lives that much more frustrating.
If you continue forward on this, you might want to join the Yahoo chat group FaceBlind and gain some insight there.