The WPS and Pacbox are such drastically different situations.
The WPS has a lot of special purpose code for scrolling the text, drawing the images, etc. A lot of things are handled independently of each other, and are dependent upon the dimensions of the screen. Drawing the WPS rotated 90 degrees would, in its simplest form, simply require the entire Rockbox think the screen was 90 degrees rotated so that the scrolling functions are suited for this.
The other option, Runtime rotation, which is the more complicated one but the only one likely to get included, would need adaptations of everything so that it can handle drawing text one way or the other, rotate the images, etc.
Pacbox on the other hand is much, MUCH simpler. The entire game expects a portrait screen. Your player has a landscape screen. You just have to rotate the image it expects to draw 90 degrees.
With the WPS, you basically need to have all the code in Rockbox be able to draw to two different shaped screens.
Honestly, it's *possible* (in my mind) that viewports could fix this. If you have a 320x240 screen, it might be possible to give a viewport a flag to swap X/Y coordinates when drawing to the "real" screen (a 90 degree rotation of the viewport) then draw everything to a 240x320 viewport. Of course, this would depend on the viewports feature supporting such a thing.