I agree that it is not possible, but for an additional reason. Even if you could book the appropriate fare code ("A" for first class and "D" for business class (I don't know what the Star RTW fare code in economy is), an RTW is a different animal and has to be priced indepently of the fare code. There are "D" fares that are not RTW fares.

I am not aware of any consumer booking engine that ever allowed an RTW to be booked at the RTW fare.