Mere moments ago, quoth I:
GitHub does have its own concept of creating releases and release notes that are *associated* with a commit while not actually being part of it -- which means that they can be edited. Though one caveat is that it's difficult to edit them ahead of time.
Perhaps this could be used -- as part of the release process, GH's API is used to generate a release (linked to the commit that's actually in the release, with some initial default content that's either autogenerated from commit messages or reads some changelog file that's part of the repository). The library maintainer can edit this after the fact if they need to. The website displays the composite of the release notes from the various library releases (as edited).
Actually, looks like they've improved things since I last looked -- you can write draft release notes ahead of time, and they can also auto-generate from commits for you if you want to be lazy (or have a starting point to elaborate on). https://docs.github.com/en/repositories/releasing-projects-on-github/automat... So the release process could probably just use an existing draft if there is one, or trigger automatic generation if there isn't. Either way, the maintainer can then edit it after the fact as needed. And perhaps the release notes on the main boost repo could be the composite (updated by a simple timer, if it's hard to detect an individual library's notes being edited), and then the website could just link to those directly. Quite simple.