Today, both CentreSIS and FocusSIS have a Moodle integration you just can't get. Oops. I couldn't find the current FocusSIS codebase. CentreSIS asked for my email, and sent a series of emails, one per day, for a couple of weeks; the second or third in the series had the link to the actual download.
I finally found RosarioSIS, yet another fork, which seems to be reasonably well maintained, and published on github. It does have a Moodle integration. It's not perfect, but it's a great start; and it uses Moodle's web services.
So, kudos to the RosarioSIS dev team, and some visibility here.
Git repo at github - https://github.com/francoisjacquet/rosariosis
And how to enable the Moodle integration - https://github.com/francoisjacquet/rosariosis/wiki/Moodle-integrator-setup
Thank you Martin for mentioning RosarioSIS. It is indeed another fork of Centre/SIS, but intended to be a really free Student Information System (no premium version).
Speaking of the Moodle integration, it is a one way integration in XMLRPC, that will cover the following aspects for the moment: http://www.rosariosis.org/RosarioSIS_presentation/#/Moodle
Contributions are welcome