A mock panel is a great idea. They can provide a valuable, fresh perspective to the interview team’s efforts.
However, it is not a good idea to invite a mock panel to view the presentation and give feedback the day before the interview. It can easily throw everything and everyone into a tail spin.
Instead, bring them in at least two days before the interview for initial feedback and then MAYBE the day before to provide feedback on the improvements made.
Some folks will argue that the team and presentation won’t be ready for an audience two days before the interview. That is OK. Get them as ready as they can be but know that the presentation and presenters are still in the development stage. You want the mock panel there to give insight and feedback that will help the team do better in front of the client. And the team needs to have sufficient time to make the adjustments and practice communicating the refinements. Some presenters require more time than others to incorporate modifications so it is imperative you consider that when scheduling your mock panel.