You're right, danrupe. An agent wouldn't necessarily know that a writer had self-published if pseudonyms were involved. But starting an agent-author relationship with a deception really isn't a good idea, even if it's possible.
I can understand why agents would baulk at representing an author who had a strongly anti-trade publishing stance, and why trade publishers would be reluctant to sign them up. In fact, a couple of years ago I passed a great self-published book to an agent I knew would love it. The author had told me that he was looking for an agent, so I didn't think that I was being inappropriate. The agent read a few pages of the book, loved it, then looked at the author's website, and that was that. He'd written so scathingly about trade publishing that she felt that she couldn't work with him. And having read his website and blog, I can understand why.