Good one!
I don't have a working approach, as I've never needed those but got me thinking; and I don't think it can be done easily, or at least without some GDL programing. Since you are talking about the 3D model (not making a 3D document and adding notes there, right?) the cloud or note would need to be fixed in space near the noted elements.
My fast solution would be to use either slabs or the cube object with a semi-transparent material to overlay in the model in a separate layer, and add some 3d text. If you also want a leader, there are some 3D line objects floating around.
Otherwise, I would look into other solutions. I don't know, maybe Solibri?
Best regards.
