Harold:
IMHO, without getting out the books, wall space is wall space and must conform to the NEC spacing requirements. I don't remember seeing the word 'usable'.
Occasionally, an EC will raise a debate about the wall space behind a door (>24") as to what 'use' a receptacle would be in that space. I see nothing in NEC that exempts that space.
As to the window seats, the required (or design) recept is installed. Bookcases either have the recept at the required spacing (although eventually behind books), or I remember one with a floor recept.
I'll consult the '08 Handbook time permitting.
