In the PC world this would be known as "BIOS modding".

The first two instructions looked legitimate, but the third looked unlikely to be a real instruction.

Given that the first appears to be a branch, that's not surprising. When disassembling, not following the flow will likely not give you anything meaningful. If the author is reading this: have you tried Ghidra?

That said, this seems a lot simpler than PC BIOSes in structure, as the latter are usually written in a combination of C and Asm (I can see why no one wanted to write MIPS Asm) and are self-extracting compressed archives.