For those doing a little digging from this thread, the one road black I have found is a case filed this year. Pursuit of information regarding the 6th citation in the NationalRivers letter led me to an issue called "navigable servitude" which cited this case/filing:
American Whitewater and American Canoe Association 8:09-2665-MGL 4th circuit (2013)
The summary, on wiki (always worth investigating) states that "A federal agency can restrict individuals paddling on a stream, because paddling is a privileged not a right." As sad as I am, that sounds about accurate from how I understand law as a laymen. I don't know of any guaranteed right to floating a river that can't be encumbered by a federal agency that justifies it under stewardship. This filing seems to agree with that understanding and is a recent case.
Just a quick google search that I thought I would share.
I fall into the camp that believes we can persuade the Interior and NPS to better designate this resource in the future so as to allow boatmen to access the launch during future shutdowns. I have seen justification at other locations for that optimism. And maybe the work of a the community can expedite that change.