application_controller.rb

class ApplicationController < ActionController::Base

  def session
    @session_wrapper ||= SessionWrapper.new(request.session)
  end

end

session_wrapper.rb

class SessionWrapper < SimpleDelegator

  def initialize(session)
    super(session)
    @session = session
  end

  def mergable_users
    @session['mergable_users'] ||= Set.new
  end

end

Instead of always writing

(session['mergable_users']||=Set.new) << 123

I can now write

session.mergable_users << 123

and it will work just fine.