lforg-usernotes.user.js

(function(){
    if(typeof(Storage) === "undefined") {
        throw new Error("No Web Storage support!  Terminating script.  Try updating your browser.");
    }

    function saveUserNotes(uid,notes) {
        try {
            localStorage.setItem('lf_notes_'+uid,notes);
            return true;
        } catch(e) {
            return false;
        }
    }
    function getUserNotes(uid) {
        var userNotes = localStorage.getItem('lf_notes_'+uid);
        if(userNotes ===  null) {
            return '';
        } else {
            return userNotes;
        }
    }

    var url = window.location.href;

    // Add to user profile
    if(url.indexOf('/user-') != -1 || url.indexOf('\@') != -1 || url.indexOf('/member.php?action=profile&uid=') != -1) {
        if(document.getElementsByClassName('navigation')[0].getElementsByClassName('active')[0].textContent == "Board Message") {
            console.log("User does not exist");
        } else {
            var userID = document.getElementsByClassName("profile_usertitle")[1].textContent.split("UID: ")[1].trim();
            console.log("Got user profile [" + userID + "]");

            var noteWorkArea = document.getElementsByTagName('table')[4];
            if(noteWorkArea === undefined) {
                noteWorkArea = document.getElementsByTagName('table')[3].parentNode;
            } else {
                noteWorkArea = noteWorkArea.parentNode
            }

            var noteWorker = document.createElement('table');
            noteWorker.setAttribute('class','tborder');
            noteWorker.setAttribute('border','0');
            noteWorker.setAttribute('cellpadding','10');
            noteWorker.setAttribute('cellspacing','0');
            noteWorker.setAttribute('style','margin-bottom:20px;');
            noteWorker.innerHTML = '<tbody><tr><td colspan="2" class="thead"><strong>User Notes</strong></td></tr><tr><td class="trow1"><textarea id="noteTextArea" style="width:100%;resize:none;min-height:300px;max-height:300px;" userID="'+userID+'"></textarea></td></tr></tbody>';

            noteWorkArea.insertBefore(noteWorker,noteWorkArea.childNodes[0]);
            document.getElementById('noteTextArea').value = getUserNotes(userID);
            document.getElementById('noteTextArea').addEventListener('keyup', function(e) {
                saveUserNotes(this.getAttribute('userID'),this.value);
                console.log("Updated user notes for "+this.getAttribute('userID'));
            });
        }
    } else if(url.indexOf('/thread-') != -1 || url.indexOf('/showthread.php?tid=') != -1) {
        var posts = document.getElementById('posts').children;

        for(var i = 0;i < posts.length; ++i) {
            var userID = posts[i].getElementsByClassName('username')[0].getElementsByTagName('a')[0].getAttribute('href').split('user-')[1];
            var noteWorkArea = posts[i].getElementsByClassName('aw')[0].parentNode;

            var noteWorker = document.createElement('li');
            noteWorker.setAttribute('class','userNotes');
            noteWorker.setAttribute('style','margin-top:5px;');
            noteWorker.innerHTML = '<textarea id="noteTextArea_'+i+'" style="width:100%;resize:none;min-height:300px;max-height:300px;" userID="'+userID+'"></textarea>'

            noteWorkArea.appendChild(noteWorker);
            document.getElementById('noteTextArea_'+i).value = getUserNotes(userID);
            document.getElementById('noteTextArea_'+i).addEventListener('keyup', function(e) {
                saveUserNotes(this.getAttribute('userID'),this.value);
                console.log("Updated user notes for "+this.getAttribute('userID'));
            });
        }
    }
})()

Made this in about an hour. I got bored...