Why do you want to log missed days in the goal itself?
A separate goal for frequency, I understand. By logging missed days in the same goal, the statistics on the goal's webpage become less useful.
For instance, I've stood on the scale 2,254 times since starting my Withings goal, with a 90% variance of 0.96 kg and an average rate of not-very-much. Adding any didn't-do-it datapoints would mess with those numbers.