Could that work in principle?
Is it OK to assign copyright to the FSF without their knowledge?
No. This would not work IMHO. e.g. you are eventually trying to assign your copyright to someone else. That someone would need to be aware of it somehow! So unless there is some agreement in place this would be likely moot and without effect.
Imagine this: say you assign your copyright to me without me knowing. Then later on someone sues me for copyright and patent infringement... How long do you think a judge would wait to actually redirect the case to go after you rather than me? I guess the best of a few seconds.
So not only this NOT ok, but in doing without my knowledge you could hurt me which is neither ethical nor what you are likely trying to achieve, correct?
You do not want to hurt me, do you?
If not, is there another foundation that accepts that?
You need to have agreement in place with that someone else.
Some orgs are setup to receive copyrights such as the Linux Foundation, the Apache Foundation or the Eclipse Foundation.
Beside these, the Software Freedom Conservancy and Software in the Public Interest are examples organizations that are more general purpose "holdings" for free and open source projects.
But the main point in all cases is that what you are trying to achieve may not work out the way you think. Even if you assign your copyright somehow, you may in most case still be able to take that same private or relicense it under another license. And if your project community does not trust you or you do not trust yourself... you have IMHO another issue to solve first!
Another way that you could consider and is much simpler: do not use your name in a copyright statement. Use instead "XXX Authors" or "XXX Project" or similar where XXX is your project name. Large projects like Chromium do this routinely. No need for assignment and this makes clear that the copyright is distributed among the project contributors.
- consider using SPDX license identifiers rather than "Licensed under " as a better and cleaner shorthand. See this article for some details.
- please write well structured copyrights that I can parse! Rather than "Copyright The project authors" use "Copyright (c) YYYY-YYYY The project authors." eventually ending with "All rights reserved."
"Even if you assign your copyright somehow, you may in most case still be able to take that same private or relicense it under another license": Very interesting, as you say that's the main point (and most important part) of your answer. So would you mind detailing how?
In the majority of the cases you are never assigning all your copyright rights. You are instead granting an unlimited license to your contributions to the so called "assignee". Therefore you are still free to relicense as you please these parts.