In which case, I'm not hopeful that coaxing a console onto USB serial will work (not even sure if that'd work in normal cases either), because the USB controller may be similarly blocked.
It's sounding like something low-level, interrupt controller or DMA. `dmesg` will reveal what hardware and drivers are involved (unlike Linux, BSD doesn't "pollute" `dmesg` with userspace cruft). The NetBSD mailing lists would be my next point of call because once it gets this deep, I'm pretty much out of my depth.
