Quote:

I need to scale up to 100,000 users, and client/server will not cut it.





No system can handle 100,000 users.
Client/Server is your best bet; if it won't cut it, you'll need a time machine to go forward a few decades and fetch the technology you need.

Look for Locoweed's Tutorial for a A6 introduction to MP programming. It's not MMO and needs a lot of work (since it's just a tutorial) but it's a good start for A7.