gistfile2.rb

keywords = []  # the keywords belonging to @publication at the time

keywords_versions = Version.all :item_type => 'Keyword', :publication_id => @publication.id

keywords_versions.group_by(&:item_id).each do |item_id, versions|
  v = versions.first :conditions => ['created_at > ?', (@publication.created_at - 3.seconds)]
  keywords << (v ? v.reify : Keyword.find(item_id))
end

variables = keywords.map do |keyword|
  variable = keyword.variable
  # Handle situation where variable has since been destroyed
  unless variable
    variable = Version.last(:item_type => 'Variable', :item_id => keyword.variable_id).reify
  end
  variable
end

gistfile3.rb

<% variables.each do |variable| %>
  <%= variable.name %><br>
<% end %>

keyword.rb

class Keyword < ActiveRecord::Base
  has_paper_trail :meta => {:publication_id => Proc.new { |keyword| keyword.publication_id }}
end