How to run disk utility on secondary display when booting from DVD?

I have an early 2008 iMac with a bad display. The cost to replace the display is unreasonable ($600-$800), so I want to get rid of the computer now. However, I want to securely erase the hard drive first. I have an external display plugged into the mini-DVI port. It works fine when the full OS (Mountain Lion) is running. I initially used Cmd-F1 to mirror the main display.

I boot the computer from a Snow Leopard DVD so I can run Disk Utility to erase the boot disk. It seems to load properly to what should be the initial Install OS X screen. The problem is that the computer is attempting to show that on the built-in display, which is completely unusable (just a bunch of faded vertical lines). The external display shows the Snow Leopard background. I can mouse over to it, so it is in extended display mode. I need to mirror the displays to be able to see the menu bar so I can get to Disk Utility. The major problem is that the switch/mirror display keyboard shortcut (Cmd-F1) doesn't work here!

Does anyone have any suggestion? Thanks!