i did some experiment after reading your post. i agree that if we have a checkbox (like the one to indicate task as folder) to indicate a task as notes is the best solution. I have suggested this but sadly the MLO development team thought that this is not a valid request as MLO is a task manager NOT a note program.
To achieve what you want which is, to replace the CHECKBOX with a NOTE icon, try this:
under Tool>Option>Automatic Formatting>
Automatic Formatting Rules>
01. Create a new rule
02. Condition = add rule = flag= note (you need to create a "note" FLAG
03. Click Icon button & select the icon u want to use. Make sure "use Icon" is checked.
04. Set Icon position = 0
05. Press OK.
Now, set up is done.
When u want to make a task become a note, do this.
01. ✓ Checked the box Under Properties>General>Folder
02. Flag the task with the Note Flag
you will see the task checkbox will be replace by the note icon you have chosen.
Note: This will only work on MLO windows version. The icon wont be shown on your mobile device (you will see folder icon instead of note icon on mobile)
Hope this is helpful.