Well sounds a great idea, and with some carefull planning Tiki would do the job. Sharing of the users can be achieved by a cron job between seperate "site" databases or LDAP for example.
The modding community is quite low at the moment, but this afternoon ive just been finishing a new site http://mods.tikigod.org to give us a boost. Well I certainly have a lot to go there in the next coming weeks.
What kind of modules are you looking for? We have a nice array included with Tiki at the moment, but I can russel them up for you if needed.
Let me know what you think and try playing with a tiki installation to get the feel for it.