Thanks guys for the ideas and suggestions. I think the core of what I am looking for is more a way to mark a topic (not in specific categories, can be anywhere) as a question that requires at least a solution. This mark would allow the production of a list of questions that have no solution (possibly filtered by dates but that we already can do). And this would not require admin/mod intervention and no regular check.
I understand tags could be used but they dont have the simplicity of a check box.