Hildon-Desktop (the "window manager" - the task switcher, launcher, etc..) is open source. So is Hildon-Home (the "desktop" that hosts the desktop widgets). So a "portrait desktop" is "legally" and "technically" possible, it's just a matter of somebody implementing it and dealing with the issues of portrait mode (i.e. how are the widgets re-arranged when switching from landscape to portrait? are there two different layouts? etc..).

For closed source applications, we can set a property on their windows to allow rotation (hildon-desktop does the rotation, which is open source), but we can in general not modify their screen layout. That's why Ctrl+Shift+R works even for closed-source apps like the media player - it sets the window to be rotatable in hildon-desktop, but the app itself cannot "relayout" itself if it's not written to do so.