No custom fields in the sidebar.
You can always use javascript to rewrite part of the page. The problem is, the javascript can’t modify the content until the content has loaded, so if you’re changing something at the top of the page, you’re likely to get a flicker of wrong and then right, especially if anything is slowing down the page load.
The code involved is going to look something like
document.querySelector('.your-author-class-selector').innerHTML = "New Fake Author Name"
Of course, Ghost also generates meta data with the author’s name. And ld+json schema with the author’s name. And you won’t have an author page for your faked up authors (so probably you’ll want your function to unlink the author’s name, too). You can rewrite those and maybe convince the search engines, but it’s going to be wrong for social shares, and anything else that doesn’t actually parse javascript.
Rather than kludging that together, maybe you want an easier option for making new contributors? Jon Hickman posted a while back his solution for making new contributors, which was that he basically has an import file that just contains some contributors. He loads it, edits the name (/ghost > settings > staff), and then is ready to use go, without having to go through validating email addresses, etc. Here’s that post: Creating new authors - #5 by jonhickman
That’s not the answer to the question you’re asking, but I think it’s actually the solution to the problem you’re having. 