Why are you so concerned that people do this? The way I see it:

The highest reward about chocobo racing is chocobo racing itself. 150 MGP...50 MGP...what for? To earn one million and buy a vanity mount or a card? If people want to afk for a few lazy MGPs, in the long term they'll realize they are fooling themselves, because they are losing out what is most enjoyable about chocobo racing...and that's racing.

I rarely encounter any afkers in my races, but if I ever encounter one, it's basically one competitor less (or just treat it like another clueless NPC). There's absolutely no harm in this.

It was literally the same thing with PVP wintraders. They would spend 2 weeks earning wolf marks win trading and then brag about a full lionsmane set xD, when the supposed fun from PVP should come from PVP itself...not from a silly gear you've earned artificially.

So my piece of advice, enjoy Gold Saucer, and raise your concerns when something really is gamebreaking.