Connection of the 2 device will be a problem.
USB on N900 is too power hungry for a always on connection, even if you dont charge or do anything. You have to pull the plug.
Running a wifi hotspot drains your battery too.
Bluetooth may work but maybe too slow for vnc.

Most usb pc arent optimized for power consumption.
So your idea with open keyboard too increase the battery space is good. But the camera may want an open spot in the "middle" of your back.

Before buying and messing with all that stuff it might be cheaper, and easier to just rent a server and vnc via 3g into that. Sadly does not work on bad connections.