I know what you mean, the sound of the wheels rolling over the gaps in the slabs of sidewalks, but I don't know how you would do it without killing your speed. The rails idea sounds like it would work of you could get the merge perfect, but skate tends to just slow the guy down when things like that touch your wheels.

I could be wrong though.

If noone else could help, and it was just for filming, you could maybe use the first sound clip on this link and loop it over your footage as needed.

Soundsnap Skate Sounds

This page is also great for anybody on here who may need a sound that the game just doesn't reproduce well.