It's already possible via InSim, and I do remember joining a server with a similar system once.
It has some very big flaws though IMHO:
If your opponent is considerably faster or slower than you, the "duell" is boring. Which would be the case in most races you do.
Also, you'll have to wait a ridiculously long amount of time just watching others race. Which is even more annoying if the former problem occurs. Which, as I said before, certainly will.