[…] I myself have never understood why one would want a browser extension to display an Edit button outside the viewport. It seems unappealing from a UX perspective and for me personally would likely fade into "banner blindness" and notice if it were detected and/or notice it too much if it tries to get my attention on any editable page.
Have you been able to check if screenreader software possibly does something useful with these rel="…" attributes?
For me this was always the main motivation to provide e.g. meaningful <link rel="prev"> and <link rel="next"> in addition to the pagination links that can be found in the content of a page. Not necessarily to "display buttons outside of the viewport". But for users with visual impairments that can't use a mouse, but have dozens of keyboard shortcuts memorized instead. The fact that these links are displayed in a toolbar is more secondary. What matters is that they have the same shortcuts assigned, no matter what the website is.
Even if it turns out that screenreaders are the only tools that do something with a <link rel="…">, and even if screenreaders only make up zero point whatnot percent, I believe it's worth it.
Remember, accessibility is a process, not a state you can reach.
Best Thiemo