- That's the time while the OS (iOS in this case) is preparing your application to start.
- No. You can not. (You can not do anything but specifying the image file in your configuration).
The launch time is the time when OS is preparing your app. The launch time is the time when OS is preparing your app + the time while ...Launching methods finish calls. And right your code gets an access, it (the launching image) is no more shown. However, obviously, you can just show the same image as your default image in your UI and continue doing some custom preparations. But me personally would consider that like 'wasting a chance to already start "entertaining" of users'.
P.S. This question is actually related to Xamarin pretty not much. Except, that considering Xamarin binaries size can just reduce the app launching time. But generally, many similar questions are widely available through "native iOS"/objective-c questions (even here on stackoverflow).
updated:
Actually, thanks Kirk for the correction on the details about FinishedLaunching. But I would really not recommend to play (freeze) much with that method as if your app won't respond in certain time, it will be terminated. Especially, as the freezing will be 100% just wasting time for users.