You can run javascript in code injection, and you can use that javascript to insert a new section containing your banner. If you want the banner on a page, then you could put that code injection just on the page’s code injection (in the footer, so the page will be there to modify). If you want it on an index page, you’ll have to detect the URL or some tag present only on that index page, and put the code in the site-wide footer.
If you’re handy with javascript, perhaps that’s enough of a nudge to get you started. :) If you want to hire someone, I’ve got an intern without enough work queued up for the end of the week.
You can still use the Ghost announcement bar to insert an announcement, and use the following code to control the display of the announcement bar on the page:
Put this into the Ghost settings > code injection > site header