David,
Ruby is already enabled (if you are using a font that supports it). See the fontspec package documentation. Fontspec also allows you to select a number of other options relevant to CJK typesetting -- take a look. (It doesn't address all the issues you raise, though.)
The ruby opentype font feature is of almost zero practical value, as virtually no font comes with it, and even if they did it would not necessarily be an appropriate looking fontface (furigana and main text in no way need to -or in many cases are even desired to- look the same) one often wants a higher readbility font for furigana, or even a sans-serif (Japanese "gothic") font fact instead. I know I personally still rely on the custom ruby code found in this mailing list's archives in the dicussion on ruby/furigana and Japanese typesetting simply because it lets me stick in a font change for texts where the main Japanese font is not suited to double as furigana font.
- Mike "Pomax" Kamermans nihongoresources.com -------------------------------------------------- Subscriptions, Archive, and List information, etc.: http://tug.org/mailman/listinfo/xetex
