Are you wedded to oneworld airlines for your trip? Because if you're not, you can do the whole thing for around US$2,000 in economy (including all taxes, fees, etc.). One quick, but not necessarily the least cost, option is a combination of British Airways, Virgin Atlantic, ANA, China Eastern, and Brussels Airlines.
Fare 1: Carrier VS XHEUR BRU to TYO
Passenger type ADT, round trip fare, booking code N, X
Covers BRU-LHR (Coach), LHR-NRT (Coach)
Fare 2: Carrier NH YLNH TYO to KMQ
Passenger type ADT, one-way fare, booking code Y
Covers NRT-KMQ (Coach)
Fare 3: Carrier MU NLPX30J KMQ to SHA
Passenger type ADT, round trip fare, booking code N
Covers KMQ-PVG (Coach)
Fare 4: Carrier MU XLAP60HK SHA to HKG
Passenger type ADT, round trip fare, booking code X
Covers PVG-HKG (Coach)
Fare 5: Carrier VS OLEUR HKG to BRU
Passenger type ADT, round trip fare, booking code O, S
Covers HKG-LHR (Coach), LHR-BRU (Coach)
The ITA website is useful for planning itineraries like yours:
http://matrix.itasoftware.com