Prevent homescreen/desktop from showing before my start-up app?
I can see nobody is answering this so I'll give you a long way of achieving this.   

You need to build a Launcher application, the launcher application will be blank, it simply starts the activity of your app, or preferably it just does nothing at all.  

After the blank launcher loads, the onBoot event will trigger your app to start.   

A long work around... 

To begin with,
Have a look on Stack Overflow, I found this question that fits perfectly in my opinion... 
Make my app the home screen launcher....?

This could be a good workaround as well, probably better than my original idea haha :) hope it helps.