Since this stuff is public now, here's a fun hack you can do on top of everything. Since the ARM7 runs pre-BIOS startup code in lgy-process9, you can actually control it quite early on. Because of that, it's possible to restore the original GBA boot screen by modifying AGB_FIRM.
Here's how I did it, in LGY ARM9: 000001EF -> 000026EF (just scan for this, it's unique)
This changes a SWI 0x1 (RegisterRamReset) used during boot into a SWI 0x26 (HardReboot).
Oh many thanks. This is going to make me tear up in nostalgia.