Can understand that argument. I am not an online racer, so I never encounter this problem. But in my mind, it might be easy to solve, by the host of the server clearly announce what track will be played, and prepare the player in advance, that if u want to race all the track on this server, u need to download this and this custom blocks in advance in order to be able to race here..... easy as that...
But again, I am not an online racer, mainly because of my unstable and slow internet connection.... so I might not be aware of everything necessary to make online racing work properly......