There is a serial port to the (ARM) processor and some folks hone tapped into it but they didn't get much further than some screen writes.
I don't know what ARM processor is being used, but in the ARM9 and ARM11 that we use at work, the speed of the serial connection is not wonderful. It's main purpose is to allow bootcode to be uploaded so that the ARM comes to life. I've never seen the insides of a JB3, but it's also quite prossible that the ARM isn't doing all the heavy lifting in which case it doubles the effort in 'hacking' it. There are SPI, UART, GPIO, and a few other interfaces into the ARM, so depending on what Creative used, there are a host of issues people would have to overcome.
At this point I think people would be better served by looking at what other devices are in the marketplace and seeing if any of the 24-bit devices meet their needs better than the aging JB3.
Wayne