There exists the multisite plugin (http://github.com/dasil003/rails-multisite/tree/master) that allows for multiple view folders based on different sites. It especially allows you to only overwrite certain views in a site and leave the others untouched (i.e. the other views don't exist twice).
Does something like this exist for ActionMailer, too? Setting ActionMailer::Base.template_root isn't enough as this has to be a string and doesn't allow an array (as ActionController::Base.view_paths does).