I think that would make it into a whole new hack, indeed

That or a severely upgraded version of this one.
If an admin just wants to limit this to certain usergroups and sizes, it would make sense to do so in a newer version of this hack.
Sharing notes is an interesting idea too, and this could indeed be a Shared Notes section in the UserCP. On RPG boards this would reduce the number of PMs going back and forth when collaborating on a post.
As for an admin viewing people's notepad, I'm not sure this is such a good idea. They can always run an SQL query if they're curious, but I'm recicent about facilitating snooping.
Usergroup Permissions:
- Allowed notes: Yes/No
- Allowed notes size: [00000] characters
- Allowed notes sharing: Yes/No
- Allowed notes number: [10] notes
This would move notes into a seperate section. If you're allowed to share notes, then each of the notes (assuming you're allowed multiple) will have its own permission fields for the user to fill out:
Note 1 of 10 < 2, 3, 4, 5, 6, 7, 8, 9, 10 >
[ Note note note ]
Can view [username1,userid2]
Can edit [username2,userid3]
[Save]