Sounds like the basis of a good idea.
I started to design a few street race map mods for GTA3, they mark out the route with flashing mines, cones and barriers. But to use them on multiplayer everyone playing would need to install them (not that it's very hard).
I do think it would be better to have some kind of standard race courses, like me said, otherwise the person laying out the course has an unfair advantage as they are the only one who knows where to go!
I think the only way to have successful 'private' races is on a private server, otherwise the other players who aren't racing are likely to get in the way at best or at worst blow you into little pieces .