Here's what I ended up doing, for the moment. I followed hanami documentation defined a custom helper and made it available to all of my views, like this:
1. Create a Web::Helpers::PathHelper module
There I can access the params and request path:
# apps/web/helpers/path_helper.rb
module Web
  module Helpers
    module PathHelper
      private
      def current_path
        params.env['REQUEST_PATH']
      end
      def current_page?(path)
        current_path == path
      end
    end
  end
end
2. Make sure the helpers directory is loaded by the app
Added the helpers path to the application load_paths variable, so that my helpers get loaded when the app loads the code.
  # apps/web/application.rb
  # Relative load paths where this application will recursively load the
  # code.
  #
  # When you add new directories, remember to add them here.
  #
  load_paths << [
    'helpers',
    'controllers',
    'views'
  ]
3. Make sure my new helper is available for each view
..by using the view.prepare block in application.rb:
  # apps/web/application.rb
  # Configure the code that will yield each time Web::View is included
  # This is useful for sharing common functionality
  #
  # See: http://www.rubydoc.info/gems/hanami-view#Configuration
  view.prepare do
    include Hanami::Helpers
    include Web::Assets::Helpers
    include Web::Helpers::PathHelper
  end
4. And now I can use my helpers in every view!
And now, from my template or my view objects, I can access my own current_path and current_page?(path) helpers, and do what I need to do with them. I don't know if that's the most straightforward way but at least it's working.