Well, dunno about foolproof - words like that just tempt fate!

You need the CARMES GIFExtractor - from www.carmes.info - that allows you to extract and import new splashes - but as I said, it screws the TMC icons - I assume that as the US chappies don't use TMC, then they haven't seen the problem.

Think you MAY be able to use NavTweek too - you can certainly see the damage any splash screen will do to the TMC icons using it.

You also need UltraISO to switch the OEM navboot file with the modified one, and something like Alcohol120% to burn the ISO image.