Oooops!
Well, it's also possible to create project/{Generic,Intel,ION,...}/splash/splash.png and get that picked up automatically by the build, which is what I was doing initially, but then while looking at the init file about NFS related stuff, I came across the support for the oemsplash.png file and thought to myself "how cooool!!!"... so... thanks for that nice little feature, and sorry for spoiling it!
I really REALLY like how modular and customizable OpenELEC is made, thanks a lot for all your hard work.
Matthias