Yeah, I read some of those posts. Bottom line is that you really don't know what the OEM has done. You can only make assumptions. Like if the board booted up and found a file of a particular name on the SDCard it would upgrade it's firmware or load in a new movie or load in a new script. You can make other guesses as well. Like it is a magnitude simpler to talk to an SDCard then it is to talk to a USB device. So it is much more likely the SDCard is being looked at then it is the USB port. Or that the bases of the firmware running on the board is Linux mostly because it is free and well documented. On that last point, if it is some sort of Linux, then there is the possibility that there is a consul port. Look for any possible connectors or places where a connector could be soldered in that appears to be a serial port. A dead give away are the silk screen letters such as "TX" & "RX". (Almost) All Unix machines run a console. Gaining access to this and finding the root password (if there even is one) would give you the ability to do just about anything with the device. Providing (big but) there is the code (or you can get the code) to do what you want.
Edit: added later
With an SD card I had laying around plugged in the unit doesn't play or anything.
Ah, that's interesting. So it does behave differently. But it does not display a thing this way? Hum, You know, SDCards formatted in modern Windows machines lack a partition table. The assumption is why bother with one as the SDCard is so small. This results in a real pain when moving from platform to platform. Also, SDCards are accessed byte by byte. But HCSDCards are accessed by (I think) sectors. Something like every address gets back a bunch of bytes on a HCSDCard. So there are two things to try right there. On top of that, you have the problem with names. I would suggest you follow what the guy said over in your other thread and use file names like 001.jpg and 002.jpg. So, I might try to find some old SDCards less then or equal to 2GB and format them on a WindowsXP machine and put files like 001.jpg and 002.jpg on them. I give it about a 1:1000 chance of working. So I wouldn't get too excited over it.