AFAIK, you could never select fare class by segment, or even direction (unless booking one ways). You had to call for that - the website would only let you select fare classes that would apply to the entire trip you are booking. However, the web site would also split into different fare classes (or break the fare) of different segments if it was cheaper then faring it together.

In the advanced search of the new site, you can still specify fare class, but to do that by segment, you still have to call.