It is not possible to disable/enable phantomjs options during runtime through poltergeist (though IIRC it is possible in general, so that feature could be added to poltergeist).
I think the easiest way to achieve this would be to register a new Capybara driver. E.g.
Capybara.register_driver :proxied_poltergeist do |app|
Capybara::Poltergeist::Driver.new(app, phantomjs_options: [...])
end
You can then use the driver where applicable by setting Capybara.current_driver = :proxied_poltergeist
. This will launch a separate phantomjs instance with the CLI options you specify.