Virtual display via VNC?

Is it possible to create a multimonitor setup where one monitor is only visible to VNC?

Context: My desk has two monitors and my MacBook Pro (mid-2010, 15") on it. One monitor is connected via a Mini Displayport to VGA adapter. The other is connected to an unused computer, on the same network as the MacBook. Since the unused computer isn't doing anything better (other than sitting on the network in case someone needs files off it) and couldn't do much that's useful (it's a Dell Precision workstation from 2002), I could very easily set up a VNC client on it to display a screen. If somehow I could convince OS X to give me a third monitor (possibly using some kind of "dummy" driver that imitates a graphics adapter with attached monitor but does not attach to any hardware), then I should presumably be able to then connect to the display using VNC and thus have a three-monitor (laptop panel plus two externals) setup without having to invest in a USB-VGA adapter.

Is this possible? If not, is there any alternative that would achieve the same aim without the investment in the additional hardware? Thanks!