Ended up doing a combo of both of the above responses. Using a default scene didn't work; for some reason the cameras were always set to the default even though I'd edited them in the saved scene. I don't think usersetup.mel runs when you open a scene, just when you load Maya, so I created a default scene with a script…
I'm 99% sure that setting a scriptJob in usersetup sticks as a global script for the entire session and doesn't get attached to the first scene in the session. I'll have to revisit that at some point. Cool that works for you though. :)
You could create a shelf button to set the default camera attributes that you can use manually. Or use a usersetup.mel with a scriptJob to do it automatically, something like: string $doCameraStuff = "do camera stuff here"; scriptJob -e "NewSceneOpened" $doCameraStuff;