I think the OP asked a slightly different question (although this is a very valuable answer). The way I read the question it was, "say I always start out with the same set of base plugins -- say engines, login_engine, user_engine, exception_notification, ez_where. Does anyone know an elegant way to press a magic button to achieve a base plugin configuration much as 'rails foo' achieves a base Rails app configuration.
use svn[1] to install the plugin - then when you install, the svn checkout will extract the external references to your plugin directory.
script/plugin install -X can be used to setup the svn externals reference, or you can use "svn propedit svn:externals vendor/plugin" to define all the external refs. The benefit you have here is you can 'freeze' to specific version of a plugin - which comes in real handy when you're mirroring production in your tests.
I think something more along these lines would be a start toward what the OP was looking for:
# Yikes! Hard coded stuff. Put your stock plugins here
{
     "exception_notification" => "http://dev.rubyonrails.org/svn/ rails/plugins/exception_notification/",
     "acts_as_authenticated" => "http://svn.techno-weenie.net/ projects/plugins/acts_as_authenticated",
     "rspec" => "svn://rubyforge.org/var/svn/rspec/ tags/REL_0_7_4/vendor/rspec_on_rails/vendor/plugins/rspec"
}.each_pair do |plugin, repos|
     force = ''
     if File.exist?("vendor/plugins/#{plugin}")
         puts "Plugin #{plugin} appears to be installed already. Force? [yN]"
         next if gets.downcase != 'y'
         force = ' --force'
     end
     system("script/plugin install #{repos} #{plugin}#{force}")
end
# ------------------------------------------------------
Clearly, this was a quick hack, but it will install a base set of plugins from their repositories. TODOs would be to install them as svn externals and/or add to repository.