Well the usual autoplay is just a program that the host computer will run when the USB flash drive is inserted, so that's useless.
But this IS possible. You need to plug the USB flash drive into a device that acts as a host controller, and that pulls of the files one by one, and sends them to the DAC. Someone actually made one for MP3 files!
It read the files off a USP key, then it had its own DAC and an FM transmitter, but the thing was tiny, plugged in to a car 12V lighter socket, and indeed was a host controller!
Here's a snip from their ad:
The host controller conforms to Full-speed (12Mb/s) USB Specification, Version 1.1
• Supports the USB flash disk device (Version 1.1 or 2.0) with Mass storage class
• Supports File System including FAT12/ FAT16/ FAT32
• Search for whole disk (MP3 File)
• Input button for Play (Pause), Previous, Next, Channel Select
Here's the url:
USB Pen Drive Car MP3 Player FM Transmitter for iPod, PDA, MP3 Player, USB Flash Drive: DJ-880 (Retail, White) (Discontinued)
I am now going to see if anyone else makes a standalone USB host controller ... certainly this is an easy DIY project.