The closest thing I found to your description is the StickyNotes plugin for Visual Studio:
It is a Visual Studio plug in that seamlessly integrates with the IDE providing sticky notes capabilities to documents within a solution.
While several methods exist for lightweight code review, such as "over the shoulder" reviews, review by email and meetings, the most effective reviews are conducted by tools to facilitate the process.
StickyNotes© gives your team an efficient and lightweight way to benefit from the power of code review with inline commenting.
With StickyNotes© your team can conduct the most efficient reviews and can fully realize the benefits of code review.
StickyNotes© can also be used for individual purposes to write reminders or todos.
While the site mentions VS 2008/2008 SP1/2010, I don't know whether 2012 is supported. You can try the trial version or contact the author. Unfortunately it's not completely free, but perhaps worth it if it'll really help you.