There is no usb wifi suppotrt in plan9 (unless you know different).

Surfing around I found a candidate for a reasonable well documented
device if anyone has the time and enthusism.

The RT2571w - a PDF of a datasheet giving register level programming is
on the net, and there is a bare metal embedded driver which might
be a fair option for example code (better than the Linux driver i suspect).

https://github.com/sbourdeauducq/rt2571w

At least there is this much info.

-Steve