I did a bit of digging and found that by selecting the "Command Line" option from the Task Manager "View > Select Columns..." entry you could see the actual command lines used to start processes. I could see that Outlook was already starting with the "/recycle" switch to reuse Outlook instances so I set out to find this in the registry.
To start Outlook with the "/nopreview" switch I edited the following registry entry. (Insert standard disclaimer here!) The registry key was:
REG_SZ: "C:\PROGRA~1\MICROS~3\Office12\Outlook.exe" /recycle /nopreview