Managed to get them to work now, I wasn't declaring using an "extern 'c'" round them (doh!).
As for what the functions are, there are loads for talking to things like the SMBus, USB ports etc. I have got a few to work (the ones that don't require parameters) that do things like reboot the box, and turn back on the secure eject system on the drive.
Unfortunately I haven't found the one yet that turns off the secure eject system on the drive, i guess you have to post some values on the SMBus, but I havent worked out what, and what the parameters are yet.
I'm starting to disassemble xboxdash, to try and work it out because it must do it, but its hard work without a disassembler that can handle xbe's.