I too do not see a way to do this.
It seems that if you want a "something" to update whenever new photos are imported into Lightroom and given the appropriate GPS coordinates within the desired geographic region, that "something" which updates is My Location.
Start a smart collection that looks for the Keyword "Home". Then, during Import, add the keyword "Home"
Clean and simple solution Joe!
Yes, this is a fine solution for "home".
However, if the original poster wants such a smart collection for photos that are within X miles of home, which My Location will do easily, I don't see how you get this in a smart collection, but using My Location works perfectly.