AdvancedInspectorでMultilineAttributeを指定したstringの配列・リストが属性反映されてないので無理やり反映させた。StringEditor.csを以下のように編集。

StringEditor.cs

using UnityEngine;
using UnityEditor;
using System;
using System.Collections;

namespace AdvancedInspector
{
    public class StringEditor : FieldEditor
    {
        public override Type[] EditedTypes
        {
            get { return new Type[] { typeof(string) }; }
        }

        public override void Draw(InspectorField field, GUIStyle style)
        {
            GUILayout.BeginHorizontal();

            float width = EditorGUIUtility.labelWidth;
            EditorGUIUtility.labelWidth = 0;

            TextFieldAttribute text = field.GetAttribute<TextFieldAttribute>();
            MultilineAttribute multiline = field.GetAttribute<MultilineAttribute>();
            TextAreaAttribute area = field.GetAttribute<TextAreaAttribute>();

            object value = GetValue(field);

            EditorGUI.BeginChangeCheck();
            GUIUtility.GetControlID(field.Path.GetHashCode(), FocusType.Passive);

            string result = "";

            // 追加した処理
#if true
            bool isArrayMultiline = false;
            if( field.Parent != null )
            {   // ParentのAttributesに指定した属性が保持されてるようなのでチェック
                var attrs = field.Parent.Attributes;
                for( int i=0; i < attrs.Length; ++i ){
                    if( attrs[i].GetType() == typeof(UnityEngine.MultilineAttribute) ){
                        isArrayMultiline = true;
                    }
                }
            }

            // ArrayかListのMultilineの場合はテキストエリアの描画処理をさせる
            if( isArrayMultiline )
            {
                if (style != null)
                    result = EditorGUILayout.TextArea((string)value, style);
                else
                    result = EditorGUILayout.TextArea((string)value);
            }
            else if ((text == null && multiline == null && area == null) || (text != null && text.Type == TextFieldType.Standard))
#else
            if ((text == null && multiline == null && area == null) || (text != null && text.Type == TextFieldType.Standard))
#endif
            {
                if (style != null)
                    result = EditorGUILayout.TextField((string)value, style);
                else
                    result = EditorGUILayout.TextField((string)value);
            }
            else if (multiline != null || area != null || text.Type == TextFieldType.Area)
            {
                if (style != null)
                    result = EditorGUILayout.TextArea((string)value, style);
                else
                    result = EditorGUILayout.TextArea((string)value);
            }
            else if (text.Type == TextFieldType.Password)
            {
                if (style != null)
                    result = EditorGUILayout.PasswordField((string)value, style);
                else
                    result = EditorGUILayout.PasswordField((string)value);
            }
            else if (text.Type == TextFieldType.Tag)
            {
                if (style != null)
                    result = EditorGUILayout.TagField((string)value, style);
                else
                    result = EditorGUILayout.TagField((string)value);
            }
            else if (text.Type == TextFieldType.File)
            {
                if (GUILayout.Button("...", GUILayout.Height(BUTTON_HEIGHT), GUILayout.Width(BUTTON_HEIGHT * 2)))
                    result = EditorUtility.OpenFilePanel(text.Title, text.Path, text.Extension);

                if (field.Mixed)
                    GUILayout.Label("---");
                else
                    GUILayout.Label((string)value);
            }
            else if (text.Type == TextFieldType.Folder)
            {
                if (GUILayout.Button("...", GUILayout.Height(BUTTON_HEIGHT), GUILayout.Width(BUTTON_HEIGHT * 2)))
                    result = EditorUtility.OpenFolderPanel(text.Title, "", "");

                if (field.Mixed)
                    GUILayout.Label("---");
                else
                    GUILayout.Label((string)value);
            }

            if (EditorGUI.EndChangeCheck())
                field.SetValue(result);

            EditorGUIUtility.labelWidth = width;

            GUILayout.EndHorizontal();
        }
    }
}