Hi,
HDCD encodes an extra 4 bits worth of information in addition to the usual 16 bit sample. I can't glean any information about how this is encoded (as HDCD's are compatible with normal non-HDCD players and so the information must be transparent to them). If you can find any extra info on the HDCD encoding process maybe it will be possible to tell whether they can be decoded effectively.
Good luck,
Andy
ps It should be noted that the HDCD standard seems to be tied to Microsoft now, which may be an indication that it offers audio qualities they'll exploit through their .WMA format... (see
HDCD info page)