I have release a project on codeplex, SPMetal Extender to be able to use additional field types. Fields in lists can be extended using partial classes. The project provides a Visual Studio plug-in vsix that generates Linq to sharePoint code for unsupport list fields such as publishing fields. Additionally the vsix deploys a new template showing how to add 4 additionaly properties to SPMetal. The Item template is guidance and needs to be adjusted by the developer after it is created.
SPMetal Extender VSIX extension installed
Add a new SPMetal Extended item to a SharePoint project
Generate code for addionnal list fields using Server Explorer using the VS IDE
SPMetal Extender is also published on Microsoft Visual Studio tools gallery