I always prefer using a Startup Script in a Group policy, Jason Sandys has written a great one,
http://blog.configmgrftw.com/configmgr-client-startup-script/ which I prefer to use. It can be used in another forest as well it has not that much to do with deploying the clent, but
you need to decied how the clients are to find the Primary Site in the untrused domain/forest.
