It's a lot lower level than drivers, without a foundation in the OS, you can't build a driver. There is a slight crossover where Mac OS runs on both NuBus and PCI architectures where USB runs on one and not the other, but probably not enough Macs to support a viable product even if it were possible.

USB took a little while to achieve ubiquity and there weren't all that many killer products available like Keychain Drives by the time NuBus Macs became wholly obsolete.