If you have a spare Android phone with enough storage, and can configure it to mount as a USB mass storage device, maybe you can install an FTP server/Syncthing client/etc. on it, use that to manage the music remotely, and plug it in to your car so that it can see the library.
I've worked on DriveDroid. There were indeed people who have done this.
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.
For many phones it is possible to change the exposed protocol from mtp to mass storage. Mass storage does need an image file with fat filesystem to work on most devices. It nowadays is not possible to expose the phones internal storage.
