@Duffy - Unfortunately this is not currently possible. The best you could probably do is to share a board in view-only mode, set the board’s start view to a spot on the board where you have instructions on how to start presentation mode, so the visitor can start your presentation.

I’d recommend adding this as an Idea in the Wish List category by following the guidelines here: Wish List: Everything You Need to Know. If you do create a Wish List Idea post, I would also recommend that you post a link to it back here, so that future readers of this post can quickly get to and vote for your Idea.