I found the "blank screen" option in the older tightvncserver 1.3.10 (lower left of Server tab), though I can't test from my virtual machine.

If that doesn't do the job, there's also an RDP client for the n900 (called rdesktop).
The Unix version of tightvnc starts a new (virtual) display, but I don't think that's possible on Windows without resorting to Remote Desktop...