One thing I know is the splash screen is removed once the the wiki is fully loaded due to a CSS. It should possibly be best left as a transient splash screen and find another way to achive what you want.
Remember a link to a wiki can include the tiddlername to load, a permalink, even a set of tiddlers and the default tiddlers also chooses what to display on start.
Perhaps you could have a tiddler that loads on startup by what ever method you wish, but one that includes styles, or a class named in the class field, that modify the wikis overall appearance until it is closed.
- This would include “display: none;” on various elements on the current tiddler and other elements on the page.
- Some of our CSS experts could possibly do this with their eyes closed
Another method is to use start-up actions to trigger a modal or window that takes over the whole screen.