Whatever it is, the Spurs have been hitting home run after home run. Doesn't appear to be merely getting "heads time after time":
Tony Parker, 2001 NBA Draft 1st Round, Pick 28
From Wikipedia:
Prior to the 2001 NBA Draft, Parker was invited to the San Antonio Spurs summer camp. Parker's first experiences were daunting as coach Gregg Popovich had him play against Spurs scout and ex-NBA player Lance Blanks. Parker was so overwhelmed by Blank's tough and physical defense that Popovich was ready to send him away after just 10 minutes. But after seeing a "best of" mix tape of Parker's best plays, Popovich decided to invite Parker a second time. This time, Parker made a better impression against Blanks; the Frenchman later described Blanks as a "one-man wrecking crew". But while Popovich decided that Parker was worth the gamble, the Spurs still had to hope that other teams would not pick Parker during the draft. As it were, Parker's name was barely mentioned in the pre-draft predictions and the point guard was drafted 28th overall by the Spurs on draft day.
Hardly a sabermetric pick, strictly an impression "eyeball" pick.
Manu Ginobili, 1999 NBA Draft 2nd Round, Pick 57
The Spurs were the originators of the "Draft and Stash" and no better example than Manu Ginobili and he did not join the Spurs until 2002. The first master of the "Eurostep", Manu could possibly be a Hall of Famer as a 6th man.
From Wikipedia:
In 2007, ESPN sportswriter John Hollinger ranked Ginóbili as the sixth best international player then-active in the NBA, describing the 57th draft pick as the "one of the great draft heists of all time", and attributed the trend of NBA teams drafting developing European players to the success of the Argentine. The following year, Ginóbili was named by ESPN as one of the best Euroleague players to have graced the NBA.
Hardly a sabermetric pick, this here is the definition of Advanced International scouting.
As for getting solid value relative to their draft position, the Spurs have made this an art:
Draft picks: Dajuan Blair(2nd Rd), Tiago Splitter(Late 1st Rd), Kawhi Leonard(Mid 1st Rd via trade)....the list goes on.
Veterans: Glenn Robinson, Nazr Mohamed, Fabricio Oberto, Rasho Nesterovic, Robert Horry, Bruce Bowen, Steve Kerr, Brent Barry, etc..
As for how this factors into Grunwald's decision making, it's very similiar. Surround the core(Melo, Stat, Chandler) with savvy solid veterans and great role players. 1 through 15 can contribute minutes at a moments notice, just like the Spurs way.