In Xcode I could easily change the Launch Screen of my App from the default Unity one to a different Image.
Is this allowed with iPhone Basic, or am I running into legal problems when deploying my app with a custom launch screen ?
You have to get pro to change the splash screen as far as I know.
Correct, you can only change the splash in Pro
If you try it with the basic license it will not load at all anymore
But I already have an App up and running on my IPad with a custom splash screen made with Unity IPhoneBasic 
So far no problems detected.
The License Comparisons say that Custom Splash Screen is only on the pro Version.
Doing it in the basic version is against legal documents even if its possible per bug.
Using the standard xcode user-interface to change the splash screen is something every app-developer can do, and has noting to do with bug-using or something like that.
If its really not allowed, then unity should restrict it somehow.
It’s illegal to do so though. That’s 100% your decision to do, and your risk to take. For example since you are in breach of the unity eula, they could decide to revoke your license.
I would keep the unity splash screen but in scene 0, make it load your splash screen. This means the unity splash screen is only visible for a second or two, but it does not fall foul of any legal issues 
Ok, I was just wondering why you can change your splashscreen without a problem.
I have this App only on my own Ipad for testing purposes, but I will add the default Unity Picture then.