MNet is a mess. It's a proprietary protocol with many known bugs, and no-one want to work on it. For some time we've talked about replacing it with standard protocols.
This needs to be done in concert with Mahara to make sure we don't break connections.
This issue is the starting point for that process.
My rough conception of the plan is as follows:
1) Identity SSO: Replace remote login with a full-featured oauth/oauth2 implementation. This will allow any Moodle site to designate any other Moodle site for authentication, as well as allowing login via Google/Twitter/Facebook etc. Mahara can authenticate using Moodle or vice versa. The user should have full knowledge and control over profile data being shared between systems.
2) Enrolment: Replace remote course enrolment by LTI at a course level. This will allow your courses on site B/C/D to appear in your site A, probably inside a special "Remote courses" category. Note this is not limited to Moodle courses, although of course we can make discovery of these more automatic. LTI can also pass the total course grade back, so we should support the storage and display of this too.
3) Portfolio services: One option here is for Mahara to expose files as a WebDAV service, and for Moodle to be able to push things to any WebDAV server via the Portfolio API.
And that should cover it.
Let's discuss the plan here, build a spec and get started ASAP.