So will your DNFs all score 18 points or will the 2nd DNF score 17 points etc. And I assume DNFs in the last 5 laps or so will count as position as per.
I can work out one GP for you though: 2005 Indianapolis;
M Schumacher 1, Barrichello 2, Monteiro 3, Karthikeyan 4, Albers 5, Friesacher 6, Trulli, Raikkonen, Button, Fisichella, Alonso, Sato, Webber, Massa, Montoya, Villeneuve, Zonta, Klien, Heidfeld, Coulthard 20 each, R.Schumacher 21.
[R. Schumacher did not make grid].
I wonder how this would affect the 2005 Championship, since Schumi would have picked up an even bigger advantage than he actually did out of the farce! (Ferrari beat Toyota in '05 by 18 points in WCC, and Trulli was on pole for this GP; Schumi would almost certainly been behind Montoya and Fisi in the WDC as well). I also think that the rule would make it impossible for drivers to make an impression on the points board should they be subbed out through injury [Think Schumi 1999], through a team dispute [Trulli 2004] or because they didn't start the season [de la Rosa, 2006].
It is very difficult to find a working scenario for this!