Found it. The route will show up some day on the next map release. Meanwhile, here's a pictorial primer for other's who would like to visit.
Follow the Orange route from Spawn to Minerduck's...
... continue east along the long, lonely, lava-warmed path...
...and you'll eventually arrive at the Nether Fortress Plain (hmm... "Plane" actually; we'll have to fix the map). Note the steps down before you reach the drop-off from the double-ramp into the lava pool.
Much of the route is uncovered, so bring your ghastly umbrella, bow, or emergency dig-a-hole security pick if that's a concern. Once you get there, feel free to climb up atop the red hill to use the sturdy cobblestone panorama viewing tower for enjoying the scenic view:
And take a nice 360 degree panoramic snapshot to send to your family and friends back home:
( Click here to view full image in high-resolution )