Once we release documentation for the IDCT engine I think you should be able to use it for JPEG. I'm not 100% sure that the JPEG and MPEG2 IDCT processing is the same but I *think* it is.
EDIT -- I don't know offhand if there is an existing API for performing hardware accelerated JPEG decompression. Building the API and wiring it into the driver structure might end up being just as hard as writing the driver code itself. Maybe an extention to the XvMC API ?