Its already a known fact that piston is better than svn:externals in many ways.
Nevertheless there is no simple way to install a plugin (execute install.rb) what you initially import via piston. This patch settle it.
Now you can install plugin via piston in two steps. Following code is an example for ActiveScaffold:
piston import http://activescaffold.googlecode.com/svn/tags/active_scaffold vendor/plugins/active_scaffold
script/plugin install vendor/plugins/active_scaffold
Without this patch the second line will produce error - something like "Plugin "ActiveScaffold" is already installed".
With patch - its just perform file "install.rb" as it need to be.