Says the guy with tiny feet.
What board company list the width at the insert/binding positions? I'll answer for you, none.
To say waist width is irrelevant is wrong. It is something that every guy with big feet looks at, that is a fact (I know I have size 13's). Combine waist width with tip/tail width and side cut its not exact but it will give you a good idea about the width of the overall board.
Not clear why you think that I have 'tiny feet' - or, in fact, what my (or your) foot size has to do with this.
You are wrong to suggest that "that every guy with big feet looks at" waist width. If they have sense, they consider width at the inserts.
The reason that manufacturers do not list width at the inserts is simply (and obviously) that the width varies along the board/for the different inserts, so that there is no single number.
The approach of taking the average of waist width and of nose/tail width to get a rough estimate for width at the inserts is fine - and is something that I have mentioned on several times before.