yes, having the tarmac in the game is'nt difficult, having it working like in the original tracks is'nt a piece of cake.
gp4 replace all the TRACKxx sections in the .gp4 with the corresponding sections in the .bin file, this allow some nice effects like skids and wet reflections.
All the tarmac sections in the .gp4 file must be in the "none" class and named TRACKxx, using different name or class cause the lack of the bin effects and the tarmac become a static object (like just in f1ch). Of course you can't have the tarmac in the .gp4 file made in one big section and named as TRACK00 or you can't see large parts of the tarmac in the game.
The unk values in the TRACKxx sections must contain some data, usually the same values of the tarmac sections of the track used as base for unk1 and unk2, 0 for unk3 and unk4 and a value for unk5 (not 0). i still don't understand what's the unk values stands for but they are source of trouble if all are set to 0.
slicing the tarmac in sections in zmod is not so difficult, i said all the TRACKxx sections have to match the corresponding sections in the .bin, 3deditor generated .bins are made of group of 60 vertices, so in zmod the exported dat object must be detached in groups of the same lenght. be sure to start by the right triangle.
For my shanghai track i've sliced all the verges too in the same number of sections, this is a damn hard work but you will be surprised when i could release it.