I believe one of the issues is that normally the logo is displayed before Rockbox itself has disk access.
I think the process is "bootloader loads Rockbox firmware image" (which contains the logo data), then Rockbox displays the logo while it does initialization stuff (which includes eventually gaining access to the disk for itself). In some cases, such as players where Rockbox can be installed to ROM the disk may not even have been spun up by the time the logo is displayed.
So custom logos would require rearranging some stuff in the boot order, or delay the display of the logo (which may result in it being displayed for almost no time at all, while the player boots without a logo or displayed anything for most of the boot time).
This is just my understanding, so I may have a few details wrong.