Let's rule out the two-headed portable as an issue. Try running it with the MBP's own LCD. If that still doesn't work, keep reading.
Click the Settings button in the IMAGINE Updater, set it to Full Screen mode and then Apply. This will undo/fix any changes made to the OutsideOptions.txt file, and try it again. Still fail? Try the other screen size in Settings.
If that fails, check the OS X control panel for what screen sizes are supported. (I find it highly unlikely that 800x600 and 1024x768 would not be listed in the control panel, but stranger things have happened.) I have found that if the screen size that is in the OutsideOption.txt file is not listed in the Monitors control panel, you will end up with a blank, white screen.
Verify that one of the OS X supported sizes is being used in the OustideOption.txt file in the SMT-I bottle. Don't make assumptions about the resolutions, either. Just because the aspect matches the monitor (i.e. 16:10) doesn't mean that the resolution works. 1440x900 does not work on my 1680x1050 iMac.