No. Slide is designed to fit or display at original scale irrespective of landscape/portrait modes. If you're using link to originals via scaled, viewer can use the pan/zoom feature to zoom into portrait original.
Currently there is no plan to make slide area scrollable only for portrait but I'll watch if there is demand.
Let me give you an example of what I mean by this. Here is portrait picture without vertical scroll, made by Fotoplayer. And Here is same picture where slide area is scrollable. I think that it's mandatory that at least portrait pictures are scrollable.
I like Fotoplayer very much. But it would be great pictures are scrollable in future.