OK let's get this straight.
Say a popular GT server with all cars, GTR, Hurracan, BMW etc. 911 etc.
I have, on numerous occasions entered servers that have the new Audi R8 GT version and the 650GT McLaren in them among all these other regular GT cars. I own neither of these cars. Yet have entered servers with them in. I know this for a fact as I was watching an onboard lap from a guy in a 650, it said so on his steering wheel boss! While I was in the server.
I have never set up a server so don't know how it works, and don't really want to, but it seems you can restrict it to anything you like. So, you might have M3 GT and 458 only, or 458 only or a smattering of GT cars.
And I am positive I have entered servers with cars I do not own in them. Why wouldn't you be able to is more the question?
As long as you cannot select them to drive yourself, they are still able to be drawn and visible in the servers and as cars you race against? The patch downloads them when they are released anyway, and you just unlock them with the purchase, so they are already there.
Not sure if I have seen it anywhere else, you pick a drift server as an example. not something I ever do so wouldn't know.
I have no idea how to take screenshots, but I will try and look tonight.