2015-10-27 Success! After at least eight hours of reading and experimenting, I managed to boot OpenBSD installer from USB Stick. (I am slow.) My Mac: PowerBook6,2 , G4 12-Inch DVI 1GHz , M9007 , A1010 , EMC 1986 . Thank you very much, Sherry Haibara, for making me aware of possibility and pointing me to instructions. My first Stick did not work: Open Firmware node usb1/disk existed, but Mac firmware could not read its contents. Every other PC I ever tried does boot from it, but I guess this Mac firmware does not like this Stick. My USB CD drive did not work: no Open Firmware node (like first Stick, it worked with every other PC). I tried a different Stick, and it worked.
(I do not find a floppy or other disc Smiley. I turned-on Rich Text Editor for nothing.)