I do not know what agreement if any Matson has with their shippers in China. If it is anything like the one Horizon (at one time) had with Maersk that would be ideal. In that arrangement Maersk basically booked the ship from China (Hong Kong & Kaohsiung) to the USWC. Horizon was allowed to use those now empty boxes (as well as their own) for use in the Hawaii/Guam trade. The intent would be for the Maersk empties to work their way back to the Far East. Again I do not know what arrangements Matson has with their China cargo shippers but I would be surprised if there wasn’t some sort of arrangement. Of course if Matson has their own boxes positioned in China, I would assume shippers would use them and the whole question is mute, logistically speaking.
As a side note; with respect to US originating cargo to Guam. The requirement is for the ships have to be US Flag, they don’t have to be US built.