I think of you are going to effectively manage a forum (and that's the big key), that can help a lot, but the nice thing about a ticket system is that customers can get an issue resolved and they know they are going to get a response.

Realmac Software (www.realmacsoftware.com) does a really nice job with their support system, especially when you consider that they have thousands of users and only a handful of employees (http://www.realmacsoftware.com/support/). They used to have the ticketing system built-into their community forums but they have since switched to a community forum area that is very helpful and that includes lots of people in the add-on community, as well as a separate direct support place that is run completely by the support staff. That aspect runs on Tender, which is a help desk backend from ENTP (http://tenderapp.com/). The nice thing about that is that it is searchable and archiveable, but you also have the ability to decide what requests or problems are kept private or public. So if you want to make every support request private by default you can. That's pretty much what 37signals is doing (Tender, that is), only with a more narrow focus and a more accessible/product specific backend.

I always like looking at what other people in maybe parallel fields are using. Get Satisfaction is common (but as an end-user I always prefer Tender-based help desks to the GS stuff) and if you could combine a way to mark an answer as ANSWERED and then tag it as an FAQ, you might effectively be able to get something from a forum that could also act as a repository.

Not sure how difficult that would be to build as a WP plugin, as I imagine it might work better with its own database as its own PHP or Python project, but that's just me.