This isn't an immediate need, just an idea that has been bugging me periodically.
Every so often I open a page and add/edit a Custom Code block, then I change pages and forget that the custom code block that's open doesn't belong to that now open page and I edit it ... ya hairbrained I know, but I'm sure I'm not the only one that does that.
What about making Custom Code blocks that belong to another page "not" editable? Only readable and copyable? That would fix it for sure! This way we can still have it open to copy code from one page to another, yet not be able to overwrite anything accidentally. Maybe a Check box that locks writability it as needed? This way we "could" edit it should we choose to, such as if a Custom Code block was part of a Linked Component system.