The link to the WGA's online list of agents on your FAQ:
http://www.wga.org/agency/AgencyList.asp goes to a list that does NOT "indicate which ones take unsolicited manuscripts.
I don't know whether the paper list they sell for a buck still indicates who take unsolicited manuscripts. I've never paid that much attention, anyway. Regardless whether an agent theoretically takes unsolicited manuscripts or not, if your query letter has a hot hook, they'll read your script. If it doesn't, they probably won't. Obviously your chances are bad at the major agencies (CAA, ICM, William Morris, UTA, Endeavor). But there's no reason not to query individual agents at even the medium agencies (Gersh, Innovative, Paradigm, etc.).
That said, if you can possibly get a personal connection to an agent so you can say, "So and so suggested I contact you," that's a big plus. But ultimately it's all about the query letter, and then the script.