Actually I'm not sure if it makes sense. If you had gh pages from a private repo, they would be public (in the sense that they are published on the open web) - so that won't solve the OP's problem.

IMO the simplest solution for the OP is to have a private repository where he does any draft work, and then pushes the master branch (or whatever) to the public repo in order to "publish".

This option is available with or without a paid account, and I don't see any significantly better option available to paid accounts. The only thing you could do there is publish your pages from a private repo, and maintain your drafts in branches in the same repo. That is virtually identical to the original suggestion except that "master branch in public repo" is replaced by "master branch in private (same) repo" which probably makes little practical difference in the workflow.