You can't record directly to the hard drive, but you can offload P2 cards to it. However, you must stop shooting and go into MCR mode, so it is not an effective solution for long-term recording -- you'll need something like the Focus Enhancements FS-100 or the upcoming Cineporter for that, or a software-based solution.
What you'd be attemptitng to do is exactly why devices like the FS-100 were created. (Another option, generally considered less capable, is the CitiDisk HD).
I don't own an FS-100, but I did have a CitiDisk HD 100 GB model as a loaner for about 3 months. The CitiDisk controlled the format of the file written to it as the signal was rec'd from the firewire port on the camera. In other words, it had to be "told" whether to record in .mov, .avi, raw, or some "user defined" format (I think Edius and another format). I may be mistaken, but I think the FS-100 has a similar "requirement."
The capture device, i.e. the firewire drive, would need appropriate firmware to "tell" the drive what format to use; without such firmware a firewire drive would have no idea what format to record in. I know the CitiDisk provided a means to change the format, and I believe the FS-100 does, also.
In my experience, the previous post is correct: a firewire drive can only be used for offloading clips that have already been recorded to a P2 card.
Nice try, but I suspect it's wishful thinking that most HVX owners have daydreamed about.