8.1 from Apple's special install CD that shipped with the iMac 233 Rev A has support for USB.
8.5 on the Blue-And-White G3, same story.
USB in 8.6 is said to work much better, but I haven't personally tried it out, as on machines where I wanted to use USB a whole lot, I went all the way up to 9.x.
G4 upgrades do work in PCI Power Macs (7000/8000/9000) with 7.5 and 7.6 on them. I don't know if most need extensions to work properly or not. I believe G3 upgrades in the PCI Performas (5000/6000 series) do need an extension to work.