This has puzzled me for years now. It must be possible,
but I have not seen software to do this. Since Pacific
Microsystems patented HDCD, one would legally have to license
the right to encode, as well as decode it. Still, some
hacker MUST have figured it out. Maybe it's too esoteric
and of limited interest, though I sure like HDCD.
I've seen a few CD burners (hardware) that say they can
rip HDCDs, presumably just as copies. But the actual
data encoding would not be done by such a machine.
Good luck and keep us posted if you find any solution.
TJ