From my own experience, small additions or clarifications, such as a text box (sticky note) appearing on a slide should be no problem if that is how your trainers want to add value to their courses. They'll surely have additional commentary or experiences to share as experts, and the visual cues can help them remember to go over them in class.
I would confirm that: >Trainers are not altering the actual content itself, only making additions >It is made clear these additions are coming from the trainer/partner, and not from Microsoft (maybe in your sticky note, the top can be labeled something like "Note from Trainer:" >You could consider additional slides to accomplish this, where you might show images or experiences from your trainer
Please note this is my understanding of the policies regarding MOC. Could either @DanDonohue or @Mindy_Rosenthal confirm this is the correct way to make additions while maintaining compliance?