I noticed something interesting in Maryland on the National Road. There's a point west of Baltimore where MD 144 ends in the middle of a National Road segment that doesn't have a point on the National Road.
I'd be interested to see what's not already covered from the routes in New York Scenic Byways: https://www.dot.ny.gov/display/programs/scenic-byways/lists
I'm guessing its going to be similar to what @neroute2 found for All-American Roads, that it would be almost all concurrent with routes we already have.
I thought this system was going to be mainly multi-state routes, not getting down into the weeds of what every state considers a scenic byway? And unfortunately I'm unaware of any GIS-based map of these things that would make it easy to pin down exact alignments. There's also the fact that many of these aren't traditional routes that go from point A to point B... they branch and have spurs all over the place (ugh).
There is a surprisingly large amount of scenic byway mileage in NY that isn't concurrent with the touring route system. I swear, if you make me drive all the way out to far eastern Long Island again... looks like I'm losing my 100% of NY for the next several
years at this rate.