MCM extended backsets possible source :)
I have found a company that makes extended backset door hardware in one of their lines.
Sargent Hardware, in their 8-line, can be ordered in a 5" backset, but if you look at the 8-line page of this distributor, it *can *also be ordered in 5" , 7", 12", 15" and 18" --including with a square rose, and in a number of finishes. So, this means if you want the center doorknob on a 24", 30" or 36" door, you can have it.
I have NO IDEA if there is a minimum.
Here is a link that might be useful: Sargent Hardware 8-line