SSO between these systems is annoyingly simple to do in Drupal, Joomla, Moodle and others, by using the CAS project - Central Authentication Server.
We are able to make SSO work with ANY systems we build, in ANY programming languages we want. This already works between several systems like:
They all use the phpCAS library:
Is there any addon / internal support for CAS on XenForo? If not, may I suggest that you do this, as it would answer many SSO/integration requests?