You're absolutely right!
I checked the MobileEngine.ini and found this:
Now, I'm guessing DefaultSplashTime and SplashTime_CastleLoad are relevant.
;Startup is already playing, so just add a looping movie to the end
;Allow for skipping the Startup movie after engine has loaded
However, manipulating those variables seem to have no effect and the UDK Startup intro length is still 30 seconds-ish when I have a custom intro movie added.
I tried setting them both to 2.0 and 10.0, but didn't notice any difference.
My current IPhoneEngine.ini: