Is there a way to allow people to set up and use .ssu.lt
You could perhaps do it within MediaWiki by writing an extension to manage it; at a minimum I think you would need to create a new database table to store the information. You could use the (unofficial) PHP bindings for the Linode API to add/remove/change subdomains.
I've made a domain look up for it, which could be connected to by such a web application for users to add contact information for their domains. I've heard something about making your own whois server the way you can make a nameserver, which might be preferable to just a web database. I can also hide various fields for the contact information; I might leave out the entire billing section since it's not needed for a free domain name.
One thing that I might add to the application would be a way to approve accounts before they can register domains after they give a reason so that I can prevent spam on a free service and keep it to domains for Warhammer 40,000 websites.
I just made a project on GitHub for anyone who has any interest in working on it.