You can use 4.1/4.2 with Visual Studio 2012
I can confirm this working. After installing the MF 4.3 SDK, Visual Studio 2012 shows all of the .NET Micro Framework versions, so it must include the multi-targeting pack all the way back to 4.2 and 4.1.
I was also able to open my old Netduino project and build+deploy+debug with no problem.
The only thing missing are the Netduino-specific project templates, which you can copy from MyDocuments\Visual Studio 2010\Templates\ProjectTemplates to MyDocuments\Visual Studio 2012\Templates\ProjectTemplates.