config-initializers-sunspot.rb

require 'sunspot_ext/resque_session_proxy'
require 'sunspot_ext/resque_index_job'

Sunspot.session = Sunspot::ResqueSessionProxy.new(Sunspot.session)

resque_index_job.rb

module Sunspot
  class IndexJob

    @queue = :sunspot_indexer

    def self.perform(*args)
      objects = []
      args.each_slice(2) do |(clazz, id)|
        object = clazz.constantize.find_by_id(id)
        # don't blow up if the object no longer exists in the db
        if object
          objects << object
        end
      end
      Sunspot.session.original_session.index(*objects)
    end

  end
end

resque_session_proxy.rb

module Sunspot
  class ResqueSessionProxy < Sunspot::SessionProxy::AbstractSessionProxy

    attr_reader :original_session

    delegate :config, :delete_dirty?, :dirty?,
              :new_search, :search,
              :new_more_like_this, :more_like_this,
              :remove, :remove!,
              :remove_by_id, :remove_by_id!,
              :remove_all, :remove_all!,
              :batch, :commit, :commit_if_delete_dirty, :commit_if_dirty,
              :index!,
              :to => :session

    def initialize(session)
      @original_session = session
    end

    alias_method :session, :original_session

    def index(*objects)
      args = []
      objects.flatten.compact.each do |object|
        args << object.class.name << object.id
      end
      Resque.enqueue(Sunspot::IndexJob, *args) unless args.empty?
    end

  end
end

Fantastic. Got this running on Heroku very quickly.