Lounge

I kind of did the same thing to distinguish the final examination period from the rest of the semester. In this case, I have extra weeks in the course so that there's a clear line between the regular semester and the finals. Inelegant but somewhat functional.
From your message, it looks as if I'm not the only one who would like to be able to create activities (especially assignments) outside of the topic/week list. In fact, the interface could make this easy in the appropriate activities sections. For instance, you create a new assignment in the page listing all the assignments, you change the dates as you please, and you decide where it should appear in the topic/week list. There could be a student-invisible week/topic for general assignments.

A case in point. I have my students in upper-level courses do a small number of critical reviews based on required readings during the semester. They choose which texts they want to write critical reviews on. So, the CR assignments are undated. I put them in "Week 0," but this method is quite inelegant and rather confusing.

The more I think about it, the more an "invisible week" makes sense... Maybe it doesn't make sense for others, but I could see some possibilities.