I have measured the area under the motherboard at the modem end. There is just a hair over 11mm available, so a 9.5mm or 8.5mm drive should fit. keep in mind that you will need an insulating sheet of some material to keep the drive from shorting out the underside of the pc board. Your local hobby and model shop should be able to supply very thin sheets of styrene plastic which should be ideal. If the drive is mounted electronics up (next to the pc board) with a sheet of plastic for insulation, the drive should disperse some of its heat through the metal shield. Another problem is that there are some plastic posts supporting the pc board and a pair of them are going to be in the way. So it looks like they will need to be cut off and relocated a little more toward the center of the pc card. And we need to find a way to keep the drive from shifting around - maybe just some little rubber feet on the pc board side to wedge the drive tight against the case.
Another possibility would be some sort of case that mounts on the bottom of the base - say 6.5" x 8" x .75" - it would extend the size of the whole unit, but would provide room for a pair of hard drives (17mm) and maybe a USB hub ot a USB Ethernet adaptor. It could be permanently attached or just velcroed in place - and it would eliminate the possibility of heat problems.
I have an 8.5mm Hitachi drive (6GB) on order from ComputerGeeks.com and I have ordered cable building materials, so as soon as they arrive I will let everyone know how it works out.