just a note: i've created a working multi-site modification of my own, based on XOOPS 2.0.7.3
this is a full modification, multiple files are affected.
you install the site from scratch, and are given the option to define the prefix for shared tables (default is 'xoops_all'), and define the unshared table prefix (default is 'xoops' like usual)
the tables that will be shared, are defined manually in mainfile.php, several are defined by default. installation commences, and all tables defined in mainfile.php are given the share prefix.
to create a second site, you clone all the 'xoops' prefixed tables, and reinsert them as 'xoops2' or something similar, copy the mainfile.php and define the new prefix (and site location, etc)
it works well, but is a bit of a manual process. i'm currently working on modifying the install procedure a bit further, so that if shared tables are detected as existing, it won't halt the install procedure (so, you can just install XOOPS like normal, with that modification)
i'm also considering implementing something to allow administrative control over which modules/module tables get shared as well. (this can be done now, but requires adding the table names to mainfile.php)
by default, groups, ranks, smilies, users, and a few others, are shared, which meets my needs, but i'd like to make this a more fluid process before releasing any files to the public