Easiest would be to voluntarily downgrade the first segment (or chose a flight that has no D booking class availability. But why do this? Presumably the HKG-ORD has to be in Y due to reimbursement rules or employer's rules about class of travel for certain purposes. A RTW by its very nature can't easily be split out into this much fare for this leg and this much for the rest. RTWs generally save money over the alternative of series of one-ways or round trips, so could the rules be loosened to allow business class HKG-ORD?

Another, more expensive, option is to buy an economy round the world, and upgrade as many of the legs (other than HKG-ORD) as you can using miles/vouchers. This is probably very expensive and depends on upgrade availability (as well as having sufficient miles/vouchers).