Afaik the only way something can be marked is by adding a blip to it. Race is already marked with the trophy blip before it starts. Instead changing the blip to something else would be helpful.
Currently the silver trophy blip, like most others, shows up on your mini map when you're in close proximity to it. Instead of coming up with another icon to locate an already famous icon what we could do is that when you use the command
/racetime * it automatically makes it so that wherever the hell in San Andreas you are your mini map will always show the silver cup blip. In addition to that a gps marker automatically starts over your head, pin pointing into the exact location of the race, much like how the electrician job auto sets a gps waypoint.
Explanation for the * : I am indeed suggesting that instead of this new command why not incorporate all of it into the /racetime one. Whenever someone uses that it's certain that he's willing to race so finding it does not need a separate command. More over the less the commands the easier it is for people to help others out. Also, if you're concerned why would I need a gps over my head when I'm already at the race - which is the case if you're already there and are using /racetime as a reference to when the race will begin.. well let me tell you that it won't activate if you're that near a race in the first place.
