Actually, logged users can add comments on presentations inside Aha! but it may be interesting to allow non-Aha! users to comment a published presentation (public URL, restricted by password or not). Visually, it may be simply a Comments icon over the presentation webpage opening a popover view with the comments and an Add Comment option. I may be as simple as providing two inputs: name and comment.
This would allow clients to provide feedback on the slides and also managers not really accustomed to Aha! to do the same.