Per the Shortcut Reference page, you can use "PreviousPicture" and "NextPicture" with PowerPoint inputs. As for changing it to a specific page, perhaps using the SelectedIndex function would work? The documentation says specifically that it works with lists, photos, and virtual sets, but since it uses the same Slideshow Settings window that Photos do, it should hopefully work the same way. Unfortunately I do not have PowerPoint installed to be able to test it for you, but it is worth trying nonetheless.