8b10b encoding: Calculating running disparity for multiple words
I found an answer in this paper on page 10/11. 

http://domino.watson.ibm.com/library/CyberDig.nsf/papers/F2DF32762A248AE685256F48006B24E6/$File/rc23408.pdf

Running disparity is calculated at byte level and chained together. With some more sophisticated logic, the 'chaining' behavior can be short circuited.