It's such a needed feature, it's possible no one is interested in developing this?
I'm new to the MVC approach XenForo is built on so for me the learning curve is a bit steep, but i don't think for someone with the right skills it should be so difficult to add a tab to the profile page visible only to admins and moderators where to leave notes.
If there's a price i think we can find an agreement, probably many ex vb users want this.