Have you looked at SqlMetal? It’s officially supported, although not promoted too much. You can use it to build dbmls from the commandline – we’ve used it as part of a db’s continous integration updates (make sure you have really good code separation if you do this though – partial classes are a saviour – as the dbml will get overwritten).
If I recall correctly it doesn’t have quite the same features as the model designer in Visual Studio (I think it handles pluralisation differently). There a good post about it on Ben Hall’s blog.