Factor 5 famously customized microcode in essentially all of their N64 games. According to some emulator devs, the F3DEX source was available in the SDK and intended to be customized [1], so it's interesting to hear a different account .
Interesting. I was on PlayStation so I only heard about N64 dev at lunch. Factor5 were much closer to Nintendo than we were, and also known to be a bit good. Plus SDKs change over time. Take PS1 for example. We didn’t get GTE register details and inline functions initially, just libraries, and the more open it became the more we wrote in assembly.