I ran NBI 0.8.3 final selecting the USB drive as the one to be updated when I installed 10.6.2 on it just the way one would for SSD
and following the install there is a copy of NBI and UpdateExtra etc on it as usual.
I have another drive with 10.5.8 installed cloned from my SSD
before I updated it to SL and that also works fine after running NBI on it.
On a third drive I created a 16GB partition and I have OS X 10.6.0 Retail install copied to that against which I ran NBM and it works just like a USB thumb drive does for installing.