It's not new, it was in the December 24, 2013 rule-making implementing the 2010 STCW and making other changes to national endorsements.
The 10,000 GT/GRT limit is set by law for as the maximum tonnage of OSVs, it can only be changed by Congress and is still valid.
Under the rules that became effective on March 24, 2014, national endorsements as Mate OSV issued under the new rules do not have tonnage limits, they can be used on any OSV.
National endorsements as Chief Mate (OSV) and Master (OSV) are now to be issued in two tonnage increments, Less Than 1,600 GRT/3,000 GT and for 1,600 GRT/3,000 GT or More. The 3,000 GT or More endorsement will have a tonnage limit determined by the mariner's service. If at least half of the service was on vessels over 3,000 GT, the tonnage limit will be 10,000 GRT/GT, the maximum size of OSVs (see above). If less than half of the service was on vessels of at least 1,600 GRT/3,000 GT, the tonnage limit will be determined by the tonnage of the vessels served on [See 46 CFR 11.493(c) and http://www.ecfr.gov/cgi-bin/text-idx?SID=0ab0d2dc84eabddb1ba221445902d155&mc=true&node=se46.1.11_1495&rgn=div8]
Note that any endorsements issued under the old rules remain subject to the tonnage limits appearing in the MMC. For example, if your endorsement as Mate is for not more than 3,000 GT, that tonnage limit still applies until the MMC is renewed and/or the tonnage limit expressly increased. Similarly, if you have Master (OSV) endorsement limited to 1,600 GRT/3,000 GT, that tonnage will remain the maximum for the vessels you can serve as Mate on.
It's not an "end run" it is (and has always been) consistent with 46 CFR 11.404(a)(1), this regulation was NOT changed in the 2013 rule-making. This regulation provides two options for Master Unlimited:
One year of service as chief mate on ocean self-propelled vessels; OR
While holding a license or MMC endorsement as chief mate of ocean self-propelled vessels of unlimited tonnage, 12 months of service on deck as follows...[includes at least 6 months as chief mate]
Note that only the second option requires the applicant to hold Chief Mate Unlimited, the first does not. Other than the 100 GRT minimum for any unlimited endorsement, there is no tonnage requirement for the "chief mate" service, and "chief mate" will be determined by the definition 46 CFR 10.107, "...the deck officer next in rank to the master and upon whom the command of the vessel will fall in the event of incapacity of the master."
Note that at the time this regulation was drafted and until the late 1990s, OSVs did not exceed 500 GRT or 3,000 GT, thus it would not have been possible to obtain an endorsement for greater than 5,000 GT based solely on service on OSVs.