We look for additional assemblies under a directory named RazorHosts (we should probably rename this) under the project and a directory above the project directory which we assume to be the solution root.
e.g. if your directory sturcture looks like this:
We'll look for assemblies under C:\Solution\RazorHosts and C:\Solution\Project1\RazorHosts and use MEF to search for additional generators. Once you create the assembly, export it via MEF as Export("<Generator Name>", typeof(IRazorCodeTransformet)].
The generator name is what you specify as a directive in your view. If you plan on specifying the same host for all files in your app, you could specify it in a file named "razorgenerator.directives" located at project root (C:\Solution\Project1\razorgenerator.directives).
Its the same name: value format that you use in your cshtml file without the razor comment blocks surrounding it.