Well, the Z/28 wasn't a secret, even for '67.
There are still things you aren't supposed to be able to do that can be done, though.
The sales brochure didn't list black as an available interior color for my Deep Forest Green Clearcoat Metallic Mustang GTS (you were stuck with tan or gray), but the dealer selected it for me and the computer accepted it, and that's how the car showed up.
You're not supposed to be able to order a Crown Victoria Police Interceptor for private use, but that can be done, too.
But no, you don't seem to be able to get hi-po engines in base models anymore. That's why I've never been crazy about the idea of SVT.