This is something we’ve been wanting to do for a while, but your specific suggestion sounds really good. This is how the Toggl app on macOS app (at least used) to work, and would make it easy to log what you were doing. In fact, I remember liking it.
We added very simplistic manual logging in our recently released Stopwatch feature, and it has obvious need for improvement, which could include something like this.
Another thing I’d want, separate from the “What were you doing when you were AFK?” prompt: A proper bucket/event editor. Should be possible to build into our vis.js timeline, we could make gaps clickable and create events in them (by flooding an event into the selection). This would also help filling in the blanks, but would enable you to also do so after the fact.
I’m also imagining a more calendar-like timeline view in the future, somewhat similar to Google Calendar.