After some search, there are many ways to do it.
So here is a possible solution, that implies the minimal changes to Omeka and modules:
- keep the current routing to create urls, except for the route “site”, that should be a regex or a custom Zend route, that checks if the slug is empty or not, to build a url without s/:site-slug. It should take care of child routes too;
- add a controller to forward requests with an empty slug (/item, /item-set…) to standard routes (the child routes of the route “site”);
- allow one site to have an empty slug: it will be the root url;
- add some checks when creating pages for the root site or global pages (“about”).