I'm posting this comment on behalf of the co-editors of CSS3 Writing Modes and CSS3 Text (Koji Ishii, Shinyu Muarakami, and myself):
We agree with previous posters that Hangul should be oriented upright. Also, Egyptian Hieroglyphic (and its derivatives) should be oriented upright as their glyphs were not rotated when written vertically.
Yi needs more investigation from someone who knows the language. Older books are written vertically, and seem to be a rotation from the Unicode code charts. However I've seen vertical captions in horizontally-set books printed upright.
Some examples of Yi: http://fantasai.inkedblade.net/style/scans/LoC008.png http://fantasai.inkedblade.net/style/scans/LoC011.png
Typeset Yi with vertical captions: http://fantasai.inkedblade.net/style/scans/LoC068