unto做介词,在这里是“已经作出的事”或“给予某人的”意思.
"Unto" is a literary,old-fashioned word that indicates something is done or given to somebody.
LZ分析的没错,这应该是介词短语作表语.如果按照语法排列,这句话是如下顺序:
The Japanese,too,are unto themselves a rule when it comes to facial expressions.
但是为了强调rule,而颠倒了一下顺序.