What I find strange seeing this discussion is that Rothgar did not come here and simply state that this window should not be modded and instead leaves you guys to waste time trying to find out what's going on.
On the other hand, since there's no official ban I suppose I can give you a hint how you can still auto-hide such a window. Remember the client keeps two copies of the UI in memory, the copy of the XML version and the working copy of all windows that have already been loaded. Now to disable (edit: or mod) the welcome screen simply modify the memory copy of the XML version via UI scripting when the interface loads. When the client then loads the welcome screen it will load the version you modified.
In case that wasn't clear enough take a look at ProfitUI's startup code, it already works with both UI copies so you should be able to figure it out from there.
