Well, Forza 2 does have several Ferraris in the game. I can't recall if the engines can be upgraded at all, but most of them can have aero nose splitters and tail spoilers added to give them a nice racecar feel. And all of them can be painted. And yes, some of them look hideous. Oh well.
And look, I hope you are insisting that no one should be able to create a custom livery on their car because a handful of kids are ricer freeks and want to do some kind of stupid hack job. Do you refuse to drive in real life because you might see some kind of a rice mobile on the road?
And this is especially strange considering you insist, "It'll be fun if we can customize our helmets. But it'll be bad if we can copy a well known driver's lid."
I mean... WTH? Customize helmets but
not cars?? You can avert your eyes, or quit the server if it gives you such an upset stomach.
And maybe just a bit more discussion:
But helmets are neutral territory. PD wouldn't have to make exceptions to it because all racing helmets are customised. An just like in real life, helmets in GT5 would be used to identify other players. *snip*
Don't get me wrong, I love the idea of custom liveries. I'd just love it a whole lot less when I start seeing Ferraris that look like they belong in a Fast and the Furious "film" (I use the term both loosely and sparingly). I admit it is the same thing with helmets because I hate seeing everyone running with Schmacher-esque or Hamilton-inspired lids, but blocking them from being used would be far easier than blocking tricked-out liveries.
On that, I think you must have eagle eyes or something if you really do look at racing helmets in-game to assess who is who. In race, you can hardly ever see them, and as icons, they wouldn't be much more distinguishable than a car icon. But in a buddy list, I doubt you'd see anything but my racing nick.
And true, if I created a nice paint job and someone else copied it, or even improved on it like happens quite a bit in Forza, oh well, that's life. But in online races, I've never seen two cars with the same livery. And even then, the nick is what matters to people, because very few have just one livery they race with. In my own case, I'm working on - or was, really, haven't touched it since Prologue

- a specific livery style to identify me for all "my" cars. And if someone thought that was cool and copied it, hey, it's a free world and all. But it won't send me running back to the paint shop to come up with a new dress to wear to the prom.
And, you know, buddy lists, it'll be coming.
As for blocking custom liveries, that's a doddle, as the British say, as simple as implementing a filter and giving it a switch for you.