If you modify your file pattern ...
landingpage_seo_templates = Dir['source/landingpages/seo/**/*.erb']
... you should get all erb
templates in the seo
tree.
You then need to modify the tpl_name
computation (there probably is a smarter/shorter way for that):
# point all landingpage/seo templates to the root
landingpage_seo_templates.map! do |tpl_name|
tpl_name = tpl_name.gsub(/.erb$/, '')
tpl_name = tpl_name.gsub(/source\/landingpages\/seo\//, '')
proxy "/#{tpl_name}/index.html", "/landingpages/seo/#{tpl_name}.html", :ignore => true
end