I'd also love the background gallery functionality from 7.0 to be added back into 7.1. Adding static text or other content over a slideshow background is such a common request.
Anyway, after much trial and error I've found a solution that - while not perfect - does the trick.
Create a gallery section. Set the gallery type to to Slideshow:Full. Set the section height, hide the controls and turn autoplay on.
Create a second section that sits below the gallery section. Set the section height the same as the gallery section. Don't upload any background images, videos or art. Add your text/button/content.
Add the following code to CSS, replacing the section id number with those of your second section. If you don't already have a Chrome extension to help you quickly see section and block ID's, I can recommend Squarespace ID Finder by Heather Tovey.
body:not(.sqs-edit-mode-active) section[data-section-id="60bd6308fec0673d7ddd4a41"] {
position: relative !important;
margin-top: -100vh !important;
background-color: transparent !important;
.section-background {
background-color: transparent !important;
}
}
Basically, it removes any background colour from your content section and moves it up to sit over the gallery section. It doesn't apply in editing mode, so all content remains easily editable. It should work on all screen sizes, but let me know if it's not scaling correctly.
It's a hack that we shouldn't need - but for now, I hope it helps someone.