So, the NMI looks like this:
> The frame begins with horizontal mirroring set (done in previous frame).
> Scrolling is set for the sky's parallax.
> Game logic runs while the sky renders.
> When finished executing logic, jump to $0600
> Write first 4 bits to MMC1 mirroring control, preparing to set vertical mirroring (but not yet).
> Spin until sprite 0 hit.
> Sprite 0 is hit (X = #$3E, Y = ~#$74)
> Timed 1-line loops, each writes 2006/2006/2005 in series.
> Timed 2-line loops, the first writes 2006/2006/2005, the second writes 2005
> Write last bit to MMC1 to select vertical mirroring.
> Set up scroll for the bottom section of the road.
> Timed 1-line loop, each writes 2005 (with immediate 2002 after).
> Set MMC1 for horizontal mirroring.
> Set scrolling for the HUD.
> Spin until next NMI.
The arrangement of the nametable is very interesting, and an integral part of the plan. It's essentially static, except for the HUD. It always looks like this:

http://rainwarrior.ca/projects/nes/radr ... etable.png
The sky obviously is meant to wrap horizontally, just a big page that scrolls around on its own for a nice parallax effect, and to put something interesting on the screen while the game executes its logic.
After the sky, there is a series of 1 or 2 line segments. Notice that all the road tiles are arranged into 2-line segments in the second page of the nametable. This controls the appearance of the height curve of the road, 1-line loops give a low slope, 2-line loops give a higher slope (possibly there is a concept here of "0-line" skips for a very low slope). Also note that the area off to the side of the road is a single colour; this way the scroll writes can happen in a wide range of space without causing visible glitches. Writes are usually near the end of a line (Nintendulator is telling me ~219 for the 2005 write in most cases; though I think there is logic in there that times it differently for when the road curves significantly left or right).
Next, the bottom segment of the road is hit. You can see how it is arranged nicely for horizontal wrapping. So, immediately the mirroring is changed, and now it controlls everything through the fine X scroll (2005).
Finally we reset back to horizontal mirroring in preparation for the next frame, and setup the scrolling to display the HUD before finishing out the frame.
The red/white road edges are animated with the palette; they do not change in the nametable.
Another interesting thing is that the render code is stored in RAM at $0600-07FF. The line loops actually overwrite the immediate values in the code that are going to be written to 2006/2005 in the code to set up for the next loop.
Also, it's notable that this isn't really glitch free, but it's designed in such a way that it responds pretty robustly to timing errors. I mentioned the sides of the screen being being a flat colour to help this, but also since the scroll usually only changes by a pixel or two per line, an error here won't usually look too out of place (especially since it's changing every frame anyway). There's actually a very noticable glitch at the switch to vertical mirroring, which shows up very prominently on the right hand side for me on the NES, in Nintendulator it doesn't show up as strongly but you can see it if you get the road turned to the right a bit.
I missed what happens with the bottom 8 lines of the sky; they weren't visible in the frame I traced. Basically it has its own horiziontal scroll, different from the rest of the sky. I presume there is another timed loop that waits until the first line of road.
Anyhow, I hope some of you enjoy this. I sure did. Sorry Bregalad if this mostly reiterates what you said in the other thread.