rotate a single slide in a keynote presentation

No, but it really doesn't make much sense to do in the context of a presentation.

Can you be more specific about your goal?

If you're presenting on screen, then rotating a single slide doesn't make sense unless your monitor/display is also going to rotate, too.

Conceptually you could achieve the same result by scaling the image/text on the slide to fit on screen - that's the same thing that would happen if Keynote tried to display a portrait slide on a landscape display.

Alternatively, as long as your presentation doesn't have animated content, export your slides to PDF, where you can rotate an individual page.