Ah-ha! Rapidshare is now playing ball.
I've done the necessary on a SATA hard drive, and when switched on the pogo appears to briefly access the disk and repeatedly spew things to the serial port, which is more than it was doing before. Unfortunately I've only got an oscilloscope to hand, rather than a 3.3V-friendly serial interface, so I won't be able to go any further until I've rounded up the relevant bits to make one.
Assuming I eventually manage to un-brick it, if the SATA port turns out to be usable for storage that'll make up for this unwanted steep bit of learning curve