I know you mentioned that you would have to use OS 9 for USB, but if you ever get 8.6 running, here is a link that should give you full USB mass storage device driver support under 8.6

http://docs.info.apple.com/article.html?artnum=60394

I'm finding lots of links where people say flash drive support is available with special drivers for NT 4/OS 8 but I can't find links. There is one 32MB stick that has drivers for NT 3.5.1 and 7.6.1, but that forum post was so old that no responses would load as to what brand the stick was.

In case you have a Fantom brand USB drive -- here is the OS 8.1 download.

http://downloads.zdnet.co.uk/0,1000000375,39047032s,00.htm