Hard to find specific evidence on this question, but I believe most of the details of the ministerials are public, and its main goals are public (because we're technically mid-Doha round, but we know all the basic arguments and points).

However, when a new member considers accession, they have one on one negotiations with all existing members, sometimes in private.

The common talking point is a little misleading in that it suggests that this is unusual (it's not), or that TPP will become binding before it becomes public (it won't).

The common rebuttal is a little misleading when suggesting that every deal is negotiated this way. (Not every deal, but enough so it's probably not that big a deal.)