User permissions on a per-entry basis? I have a Structure section that contains most of the pages on my website.

We have users that need to edit only some of those pages. Since permissions are set on a per-section basis, right now they can edit either all or none of those pages.

How would I go about building custom user permissions for only some of those pages?
This might be considered more of a "workaround" than a technical solution, but I think it could actually solve your problem quite nicely...

You can use CP CSS and/or CP JS to show or hide certain fields.

You'll probably find that the CP Body Classes plugin is helpful for narrowing in on specific conditions. For example, you could show/hide certain fields based on a specific user group.

Disclaimer: I wrote each of those plugins. Feel free to leave feedback on Github!