Well, I am not sure how you would do it in the Engine, but have some questions that might help others know what you are looking for.
Do you need more than one color? If not, couldn’t you just use an editable mask? Could you simply define where the user is likely to draw, and use a texture ahead of time?
A typing interface may be better if you expect notes to be longer than very short words. Or, you could give premade phrases they can stamp down on, in which case you could use texture coordinates to match a mask you have for that purpose.
Is it important that it is handwritten by the user? And, will it be done on any 3D object, which requires different strategies than a flat 2D object?