I couldn't find any concrete way of selectively pulling content from an old Wordpress blog (with attachments). So... this python script will take an xml export file of all your Wordpress content and only move over the attachments that are found based on your selective content Wordpress export.

wp_xml_export_mashup.py

import codecs
from bs4 import BeautifulSoup

soup_all = BeautifulSoup(open('Wordpress_all_content.xml'))
soup_all_items = soup_all.find_all('item')

soup_posts = BeautifulSoup(open('Wordpress_selective_content.xml'))
soup_posts_items = soup_posts.channel.find_all('item')

def match_post(item):
	for elem in soup_posts_items:
		if elem.find('wp:post_id').text == item.find('wp:post_parent').text:
			return True
	return False

for elem in soup_all_items:
	if elem.find('wp:post_type').text == 'attachment' and match_post(elem):
		soup_posts.channel.append(elem)

with codecs.open('new_wp_import.xml', 'w', 'UTF-8') as file:
	file.write(unicode(str(soup_posts), 'UTF-8'))