According to B-R.com, the HOFer with the lowest career Win Shares (I wouldn't even know how to ask it to look for lowest winning percentage) among player with at least two All-Star selections was Maurice Stokes (amusingly enough, another former King/Royal), with 16.1.
And, now, a question for you (one I already know the answer to, but I'll ask anyway): how many players in the history of the NBA with career averages of 20+ points and 10+ rebounds are eligible for the Hall of Fame, but not in? I'm telling you, man, one way or another, Cousins is going to establish new precedent.