Libretro: np2kai: where to specify font/bios files?

Hello, I am trying to run PC-98 games with the np2kai core, but text is missing presumably due to missing BIOS files (including FONT.ROM). I have the files in question, but how do I specify to the core where to find them?

OS: Arch Linux x86_64
Model: Inspiron 3558
Kernel: Linux 4.19.8-arch1-1-ARCH
Resolution: 1366x768 @ 60.00Hz
WM: i3
CPU: Intel i5-5200U (4) @ 2.100GHz
GPU: Intel HD Graphics 5500

In case anyone comes across this, I managed to figure it out several months later by running retroarch under strace. The files should go under an np2kai folder in your retroarch system directory, e.g. ${HOME}/.config/retroarch/system by default. Works great now!