You also run into the fact that once init is called, the boot process is almost over. A delay would need to be added for the logo to be seen for more then a brief second. I saw my custom logo on my Mark 1 long ago because I popped it on screen right before a 5 or so second startup sound was played.

I still think the easiest way is to flash from a program on the drive during boot if possible. Elliminates the need for a custom kernel, and custom segmentation of the flash. And the disk can hold just a few more logos then the flash