As I mentioned over PM, this is some impressive texture work here--nicely done!

Generally speaking, if a texture is included in the NAM, we're more than happy to see folks modify them to produce regional variants or textures that could be used for expansions to our existing content.
Regarding the implementation side of things, the traditional way to handle this would indeed have been a set of "Cosmetic Pieces"--essentially, conventional Static Puzzle Pieces (SPPs) used for aesthetic reasons, along the lines of what
dedgren did with the Rural Roads Plugin (RuRP), and later, what
MandelSoft did on the RHW side.
As a few users have pointed out, the NAM Team has generally been trying to avoid further proliferation of conventional static puzzle pieces, mostly because of significant ease-of-use issues that our prior
modus operandi of cranking out hundreds of puzzle pieces per release created. Since each conventional static puzzle piece can only serve a single purpose, and there are so many components in the NAM now, there's no way to accommodate future expansion except to create more SPPs, and it basically turns into "rabbit breeding".
SPPs are okay when one is dealing with something that isn't intended to exist in parallel on other base or override networks, and/or don't seem particularly conducive to a draggable or FLEX solution, and they're certainly preferable to TE Lots or things like the old "automatic" version of the Road Turning Lanes plugin, which nearly killed the Network Widening Mod, Draggable Fractional Angle Roads, and the Draggable Elevated Road Viaducts.
Based on the initial prototype, that probably could have been safe SPP territory. Once things started expanding, however, to plain Roads and Streets, plus Roads and Streets without markings, it reached the point where SPPs become unmanageable--on my count, you'd need just over 100 pieces to cover everything shown so far.
Given the types of designs here, they actually fit with some existing NAM expansion plans that have been on the backburner, namely with intersection control variants (related to the SITAP project with the One-Way Road network), with plans to make the PedMalls/paths draggable, and with NWM/Roundabout crosslinking. The only thing not really covered in those previous plans are the zigzag setups approaching the crosswalks, but those could easily be handled in a manner similar to what we're doing with the FLEX Turn Lanes, which would allow us to expand that functionality to other networks (i.e. NWM and SAM) without really needing to create any new pieces beyond the initial offering.
Hope that gives some insight into the NAM thought process.
-Alex