I use a Blueke Fire Diamond. It can boot even with no files on it at all. I don't know whether it already has this function set up by itself (so-called bootable).
But theoretically speaking, if DOS can't use a USB flash drive, then it's no use no matter what files you put on the flash drive.
Some people say that by making an AUTOEXEC.bat file and copying the driver files that were put on the USB drive beforehand into the system folder, you can make it work without needing drivers. But I think before 98 loads, DOS simply doesn't recognize USB devices at all, so how can you use batch files then?
I don't have a machine with 98 installed on hand right now, so I can't test it. Could an expert explain the principle?