no it wont command the phone. the framework and apps don't have an interface for that. plus the headunit would need special drivers if the phone had that capability. the issue with android is too many manufacturers do things different so unless google comes out with that on a nexus and tells maunfacturers the way it needs to be implimented I don't see any hope.
there might be a way to echo the commands through adb if you have a computer in your car. but I guess you'd have to know how the music app works for that.
we All know apple has the accessories market down pretty good. even though some older accessories don't work with the newest hardware. that's the one thing apple is good for, a wide accessory market.
now if you have a Bluetooth controler and it uses the right protocol to work with thw samaung bluetooth stack then you might be able to map keys to certain functions with an app or simple configuration file editing. but as far as USB goes I don't think android is designed to host USB input devices. there is limited otg usb support on certain special galaxy s and nexus 1 and nexus s kernels but nothing native.
maybe some 3rd party root app could eventually be developed but I haven't heard of one yet.
besides all that, who knows if the head unit will know to go into audio over USB mode in the first place, even if you get the phone in that mode, will the headunit be looking for USB storage and an iPhone interface only? was it designed to accept USB audio other than an iPod interface?