Isn't it just what bios...uh, header you burn. I thought some guys had made every splash screen available.
To the best of my knowledge you can't just swap the splash screen because it's deep inside the code and nobody reverse engineered it before. However all the bios roms have been dumped, and as the hardware is the same, you can just pick and choose whichever you use in your machine.
About the only "incompatibility" is that the earliest ones need a CD drive attached to work, otherwise they don't boot (while later ones boot but with misaligned cubes) - but that may be related to the CD block firmware (which is something you'd be extremely unlikely to replace), not the bios rom.
edit: which model Hi-Saturn do you have, by the way...? Is it a model one with the serial starting with BC5000...?