A flash drive has nothing to jailbreak. It's just a plain stupid storage device. If this conversation was held a decade ago you'd be asking about syncing to a floppy disk.
We don't know what Apple's policy decisions are. Apple is a for-profit company and that will likely guide their decision making. Still, I invite you to think, how might Apple answer? Many players now enable playback from iOS devices. Why the need for something extra when all you need do is plug your iPhone or iPad (both of which you own, of course) into the car?
Sure, look for an alternative to iTunes. There's a few (e.g., Songbird) but I don't see people moving to them in droves. iTunes isn't perfect but being free it has killed the competition. (Its only competition really was Soundjam which in fact became iTunes.) Until millions of people see the need to sync to a flash drive and either somebody comes up with a free or low-cost alternative, the alternatives just aren't there, sorry.
The only thing you're really missing in full syncing is the ability for iTunes to also remove content on the drive. You can set up a playlist in iTunes, then drag the whole thing to the flash drive open in a Finder Window and it will copy to the drive.
If your player supports m3u playlists, you can create those with iTunes.
Look on the web. I know several years ago I came across an Applescript to enable pseudo-syncing with a Sansa Clip player.