Have the same task. I use the DNF list from GC and check von time to time where I have DNF and someone later found he cache. When I then go finding it myself, I delete the DNF. And I use an App where I have different categories. And also a DNF. When I use the App (Android) and find the cache and categorieze it as found, I see that I already have categorized it als DNF.
But at the end it is manually and so I also think it is a very good feature request.