The reference projects needed to resolve objects that reside inside an MSDB database have been posted on the MSDN Code Gallery site.
You can find them on: http://code.msdn.microsoft.com/dbpromsdbsamples
We did not post .dbmeta files but stub projects, because .dbmeta files require the collation to match with your environment. This would result in a multitude of files do download. So instead we provided projects, you do need to make sure you set the project collation to match with your own environment. After you have done this you can create your own .dbmeta file if you do not want to reference a project.
NOTE: Since the projects are a stub projects, in other words the content of objects like procedure have been removed, you cannot and should not use these project to deploy to your msdb database!