There are login-group options which lets you have a single sign on with a group of repos, To unify the other bits it looks like it would be possible to change the main menu links to the central repo(in the web settings).

Never wanted to do what you want but it sounds possible

set up the fossil server on a directory of repos probably with --repolist to get a list

    fossil server --repolist /var/fossil/

get all repos in the same login-group

change the menus in each repo to point to the core repo. Based on some quick tests I suspect a full http:// url is required.

    - Forum     /forum       {@2 3 4 5 6}
    + Forum     https://mydomain.org/core/forum       {@2 3 4 5 6}