I have not found any information on how to do this, or even if it’s possible at all.
http://msdn.microsoft.com/en-us/library/e13syb43
does not mention anything about building a portable class library, and
http://msdn.microsoft.com/en-us/library/gg597391.aspx
does not mention anything about a command line interface.
Has anyone else tried to do this and found a solution/found it to be impossible?
Building portable libraries via the command-line is possible, you just need to pass the correct framework references. These determine what platforms you will support.
First, is the reason you want to do this via the command-line because you don’t have Visual Studio Pro? If so, it gets a little tricky.