OK, I couldn't keep my mind away from the NY scenic byways topic. The best way to determine routing would probably be whatever maps are in the Corridor Management Plans (CMPs) and linked websites and comparing those to street view for signage (as such don't always match; even the
map on the website shows loops/spurs that aren't necessarily signed, although many are). It turns out more of it is on the touring routes than I thought; I guest most of the ones on local/county roads are in my part of the state, so I had an inflated perception of that.
There are two classifications on that list: "national [sic] designated" and "state designated". The national ones include the Seaway Trail, Lakes to Locks Passage (which gets
interesting in Plattsburgh for seemingly no reason), Mohawk Towpath, and Palisades Scenic Byway (which is just the PIP and would have been a neat solution to the usasf vs. usanyp problem if it weren't for the fact that it would mean a demotion to tier 5). All of these are signed, have portions that aren't otherwise in TM (except the PIP), and are generally coherent routes (Mohawk Towpath has a signed alt route; Lakes to Locks shows spurs and loops on the map but they aren't signed).
Of the state ones (note that I only took a rough look; it might be a good idea to verify more closely):
Adirondack Trail: appears to be concurrent with existing routes
Black River Trail: concurrent with existing routes per signage; the maps show some local roads around Watertown, but it's signed on NY 3/US 11 SB/NY 12F
Blue Ridge Road: entirely on a county road (interestingly, not fully on the map, and does not have a CMP) but does not appear to have any special shield (just the street name and CR pentagon)
Bronx River Parkway: already in TM
Catskill Mountains Scenic Byway: signed portions concurrent with existing routes
Cayuga Lake Scenic Byway: concurrent with existing routes
Central Adirondack Trail: appears to be concurrent with existing routes
Dude Ranch Trail:
somewhat signed, mostly local/county roads
Durham Valley Scenic Byway:
somewhat signed, but it appears to be less a true route but a collection of local/county roads that haphazardly had the scenic byway label applied to them; most are too minor to even have street view coverage
High Peaks Byway (Route 73): concurrent with existing route
Historic Parkways of Long Island: already in TM
Maple Traditions: concurrent with existing routes
Military Trail: appears to be concurrent with existing routes
Mountain Cloves: has spurs not in TM
North Fork Trail: did not see signs in street view but would appear to include some stuff not in TM; unsure of exact routing other than it includes CR 48 and NY 25
Olympic Trail: concurrent with existing routes
Revolutionary Byway: appears to be concurrent with existing routes
Roosevelt-Marcy Trail: concurrent with existing route
Route 20 Scenic Byway: has a signed alt route on local/county roads
Scenic Route 90: concurrent with existing routes
Seneca Lake Byway: concurrent with existing routes
Shawangunk Mountains: has portions on local/county roads, but good luck finding an easy way to draft it
Southern Adirondack Trail: appears to be concurrent with existing routes
Taconic Parkway: already in TM
Upper Delaware (Route 97): concurrent with existing routes
WNY Southtowns Scenic Byway: has a signed alt route on local/county roads
I can see a case for adding the other "national designated" ones, but the state designated ones might be a big haphazard for TM purposes.
