I have to say no to this one as well.
First of all, it's pointless. The only reason they work on the 360 is because they're global. Every game has them, so you can see what other people have done, and they can see how you've done or what you've missed. It's useful for friends who want help with a particular game or section.
Having self-contained single-game achievements is pointless. Only people who play already will understand, and where would you share them? On LFSW? Which you need to already be into LFS to know about? Pointless waste of bandwidth. And how would they be displayed? A generated image, using up more bandwidth for people to share on forums where they're already talking about LFS?
On top of that, each and every action would need to be logged. So LFSW would need to be updated when you hit that barrier, and the height you flew would need to be logged just in case you get enough airtime for the achievement. So the servers would be getting pinged with useless stuff every second of every race.
It would also encourage achievement whores. The kind of person who will intentionally do nothing but clip barriers in order to get that one elusive achievement, potentially ruining a race for somebody else when they knock them out of the way or land on them.
LFSW already has a real achievement tracking system. It tells you if you've beaten your previous best time, it tracks how many races you've won. It's not worth the extra work for the sake of a few people who want to expand their e-penises.
And what would you make achivements in? Clean sections? That should be what you aim for in a race anyway, not something that deserves special merit.