Today big5 finally got fully defined in the Encoding Standard. Philip Jägenstedt emailed some more analysis on the big5-uao and big5-2003 extensions of big5. It seems the only one we want to support is big5-hkscs which is what the standard defines today, with a slightly restricted encoder.
Apart from some minor bugs the encoding part of the Encoding Standard is complete now, with work on limited/broad encoding sniffing lined up. Just got a little bit closer to fully predictable and understandable platform.