i KNOW it is possible, i have done it to my old compaq 5150.
it has only support for a 6Ghdd (it canot "see" larger) i had a second hdd on it for a while (the old 2Gb to be precise),
and yes on the CDrom port.
I got the contact soldered wires (from an old IDE cable) from each pin on that to the pins on the hdd, i then glued the whole thing to a thin piece of plexi (to get the contact seated and hdd inside the computer).
it worked well for several weeks until the old already failing drive died on me.