We're going full circle!
For the curious, this is not a hack, but running on real, official hardware and software.
More precisely, on the
F-Zero X Expansion kit for the rare and short-lived
64DD, coming with some extra tracks and a full-fledged track creator!
You can find the full making of the track
here to see how the track creator works and what it can do.
There are plenty of videos on YouTube of custom tracks made with this fantastic tool, including various Rainbow Road remakes

There have been some small tweaks made as to go around the limitations of the track builder, as well as to fit better the FZX physics, but it's really great to see how fitting the track design is when put in an actual F-Zero game! Shows even better the care that has been put by the Mario Kart 8 dev team on the DLC.