Don't know how feasible

Two problems:

1. We'd need an AAC codec. This costs money. Unless there's a strong demand for it, we're not going to get one. Also, even if we did get one (for Karma, e.g.), we'd probably need a separate license to use it on the car player.
2. We'd need to deal with the DRM used on the iTunes store. This would require a licensing agreement with Apple. Again, more money.

In short: It ain't gonna happen for the car player.