That depends on if it's a bitmap font or not. If it is bitmap, then it's already subpixel rendered, and you would need two different bitmaps depending on orientation. If it's truetype or similar, you would need a font renderer that was capable of subpixel rendering, and it would then have to render it differently depending on orientation.

However, subpixel rendering works because you effectively triple the horizontal resolution. If you flip your screen, you now have normal horizontal resolution, but triple the vertical resolution, which is less useful for rendering fonts, and it would of course also not look the same as the horizontally subpixel-rendered font.

So for devices like that, it's probably best to simply not do it and get a dense enough display that you won't need it.

Also, does anyone know if subpixel rendering works on OLED screens?