I was looking for the same answer, what I ended up doing, it is a little more than I would hope:
First, download the file from Google Webmaster Tools.
Place that file in /app/views/static and rename to .html.erb, also take note of the random text after google, we'll use it int he next steps.
Create a static_controller.rb in /app/controllers:
class StaticController < ApplicationController
def google[random code from filename here]
render :layout => false
end
end
Add this to your routes.rb in /config/:
get 'google[random code from filename here].html' => 'static#google[random code from filename here]'
This better follows the MVC setup preferred for Rails, even if it is a little more... requiring than just dropping in an html file.