@gwennelsonuk you can literally build libreboot for the playstation and install it. works in emulators and on real hardware. it uses the pcsx-redux open bios
you can use one of these boards:
https://oshpark.com/profiles/mi213
flash libreboot to a SST39VF040 or compatible 512k nor flash. you could also flash the original bios on another, and stack them; then with each one having otherwise isolated OE/CS, switch OE/CS (output enable / chip select). could wire it to a timer on the reset switch or something