The way I read the rules now for Master OSV greater than 3,000 GT makes it almost identical to the requirements for Master Unlimited, including one FULL year as chief mate. Additionally, to get the corresponding STCW endorsement they need to complete all the same classes and assessments as CM/Master unlimited (the same as for Master 1600/3000 now). Unless the USCG exempts then from requirements deemed “too burdensome” the only significant difference I see is the exams the Unlimited CM/M have to take.
Now, I expect that once this new scheme gets started the boat companies will write sea time letters as “Chief Mate” for anyone holding a Chief Mate OSV license, regardless of what their actual position onboard is, to get them Master licenses. They already write sea time as master for anyone with a master’s license and they give officer sea time to licensed lookouts.