I know the dedicated backup monitor has a 4th input that can either be a 4th camera or a typical video device over composite (yellow cable) signal.
However, there seem to be few options for routing the display of a handheld unit out to an external display. I know Jage was looking into this a while back and I think his searches led him towards some marine products.
Also, there are a few problems with the ones where the display is IN the rearview mirror that are hard to realize until you go to install one.
a: the display is a little small to really be useful
b: the wiring can be a mess.
There's not a lot of room in the overhead right there, and the cabling for my rearview monitor system had to be routed out of the roof further away from the windshield than I would have liked.
You can sorta see my rearview monitor in this picture:
It hangs from about 3 inches away from the glass. Since that pic I've removed the standard rearview mirror, as it's useless.
I believe I spent around $1500 for the monitor and all three cameras (there are two under the side mirrors for the blind spots) and it was installed by Rob and Randy of Integrated Audio Solutions (the guys at the SMB factory).
One thing I really like is that it's keyed to the turn signal to show that side's blind spot. It really does make the van less intimidating to drive in traffic.
It's a voyager system, and of all the gadgets I have in the van I'm really surprised by how much I like it more than I thought I would.