A coisa mais próxima que encontrei da sua descrição é o StickyNotes plugin para o 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.
Embora o site mencione o VS 2008/2008 SP1 / 2010, não sei se 2012 é suportado. Você pode tentar a versão de teste ou contatar o autor. Infelizmente não é completamente grátis, mas talvez valha a pena se realmente te ajudar.