The wordpress-importer plugin doesn't understand namespaces. Here's a workaround that converts XML generated by Go into XML this plugin understands.

prepare-wxr.rb

#!/bin/ruby

# Read an XML document on STDIN, write an XML document on STDOUT
#
# Replace
#   <wxr_version xmlns="http://wordpress.org/export/1.2/">1.2</wxr_version>
# with
#   <wp:wxr_version>1.2</wp:wxr_version>
# and add xmlns:wp=... to the root node

require 'nokogiri'

namespaces = {
  'excerpt' => 'http://wordpress.org/export/1.2/excerpt/',
  'content' => 'http://purl.org/rss/1.0/modules/content/',
  'wfw' => 'http://wellformedweb.org/CommentAPI/',
  'dc' => 'http://purl.org/dc/elements/1.1/',
  'wp' => 'http://wordpress.org/export/1.2/',
}

doc = Nokogiri::XML(STDIN)

namespaces.each do |k,v|
  doc.root.add_namespace_definition(k, v)
end

doc.xpath('//*').each do |x|
  namespaces.each do |k,v|
    if x.namespace && x.namespace.href == v
      x.name = k+':'+x.name
    end
  end
end

puts doc