Main post updated again; I think there's almost enough information here for a course editor now.
Here's what's on my todo list:
-Collect all hardcoded segment 6 pointers
-Draft "course script" engine to replace the hardcoded stuff
-Find course m64 references
-Find course object placement method (not item boxes)
-Find what 0x18:0x2C in course data structs are for
-Find how segment 5 is filled (course textures)
-Find how segment 7 f3d is generated
-Find what 8029D584 function does
Also, here's a pretty render of rainbow road's path data: