Thanks for the response Jeff.
I was committed to iDVD because I was using a particular template I had bought, so I was stuck with it and couldn’t jump up to DVDSP.
Your link was really useful and helped me finally resolve my ‘Where the heck is my QT Pro?’ question. Installed it from the Snow Leopard disc. Even though I then had control over the aspect ratio, it still didn’t look full 16:9 after I’d punched in the numbers. But it looks close enough.