I swear I saw someone had a custom message flash across the DIC screen at startup. May have been a C5 or C6 Corvette or something. The image in my head is foggy, but the teal green text and gauge layout was definitely GM.
Having a custom splash screen on the HU would suffice nicely! I remember back in '02-ish, I bought a then top-of-the-line single DIN Pioneer Premier that could do custom screens. You had to download a program to your computer from Pioneer's site, open and edit the pics including setting the scrolling style, burn the images to a CD, put the CD in the HU and go into a special menu to copy them from the CD to the HU's memory. It was a background feature with half-decent color scale that would scroll through the images like a sideshow, similar to "the dolphin" HUs that came out later