There are some agents such as Travelbag and Trailfinders who can quote prices and itineraries. Having said that there is a planning facility on Oneworld's website.
I am thinking of doing something similar and used Oneworld.com to look at some example itineraries. These itineraries give specific flight options on specific days which means that you can choose airlines and aircraft (eg A380) for each leg of the journey. Hope this is of some help.