As in, I'm fine with the SYSTEM that plays music now (I have Toyota Entune) and I don't much want to mess with it.
I literally just want to be able to remotely change the files on the USB drive in the car -- without ever having to take it out of the car; that's it.
Best example of such a use-case was someone who wanted to make an old printing press fetch files its files from the internet. The press only had an interface for floppy disks. He replaced the floppy disk controller with a floppy emulator that exposed an USB port. Connected a phone with DriveDroid. Synced usb image files from the internet to be exposed over usb mass storage. The image files were fat images that hold the printing job files. They were generated by the server automatically. It worked pretty well from what I heard.
Similar solutions are also popular in the 3D printing space, look up Fysetc or BTT wifi sd cards.
Something I’d still like to add is a USB OTG emulation of iOS/Android/iPod/etc, so that the currently playing track shows on the dash, steering wheel controls can be used, etc, but my last experimentation a couple years ago didn’t go anywhere. (All the open source stuff for emulating CarPlay and Android Auto seem to be for the other direction: the dash, not the device.)