I have never seen anything but a "generic" hollow on inexpensive basses, usually left as is and sometimes pretty crude. I have seen nicely done hollowing that goes the full length of the overhang and wonder how the top luthiers approach this. Any opinions?
Don't know about any top luthiers, Hayes, but I just leave 'em the way they come to me. You could hollow out a few inches more just with a thumbplane and scraper. Not a big deal, I just don't see a need. Some would do this to lighten the neck.
AO-BO mode matching has been debated at length in the past here. It is the process of matching the resonant frequency of the air chamber (body) with the resonant frequency of the fingerboard/ neck assembly. Some believe it can make a substantial difference in the sound of an instrument. Some, like Mr. Bowlback, think it's bubbermeiser. (love that word...) A search will likely turn up some spirited discussion.