Is the CallerMemberName attribute in 4.5 "able to be faked"?

Yes, you can, exactly as you could use LINQ and .NET 2, as you said. I use the following in a .NET 4.0 project with the VS2012 compiler with success:

namespace System.Runtime.CompilerServices {

    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = true)]
    public sealed class CallerMemberNameAttribute : Attribute {
    }

}

Be very careful that everyone on the project is using VS2012, otherwise they'll get silent bugs because CallerMemberNameAttribute didn't do anything and null was passed as the parameter default.

Edit 2013-06-28: Consider installing the Microsoft.Bcl NuGet package that provides CallerMemberNameAttribute (and some other classes from .NET 4.5) for .NET 4 and Silverlight rather than doing it manually.