They certainly can and they usually are. Even though documents are formally classified, they are usually more transparent than the TPP. They do tend to be circulated, and someone who has a strong interest in these matters can usually get access to the relevant texts. That's certainly been the case in all the negotiations that I've followed closely.
If you want to look at alternative models for negotiating treaties, especially ones that, as I said, increasingly deal with regulatory matters, you could look at the UN process, say, the Convention on Biological Diversity or the Kyoto protocol. Those negotiating sessions are completely open. Commercial interests and non-governmental organizations can observe the negotiations. They're on the basis of public proposals and public texts.
I would like to see trade negotiations move more in that direction rather than the other way.