I have an old thin client that uses VIA PLE133T chipset, one 44 pin IDE header but no other boot devices. No USB boot support of course. I put Plop on a hard drive using another computer and it starts that just fine. I've tried various bootable distributions from Unetbootin on various USB flash drives, along with using Rawwrite with an ISO, but none work. By messing around with Plop's USB 1.1 mode (mode 1, mode 2) the best I can do is to get the first line displaying "SYSLINUX blah blah H. Peter Anvin" if the boot image uses SYSLINUX, although sometimes only half that line is displayed. If the boot image doesn't use SYSLINUX, like a random ISO, it freezes up at some point without showing any progress of actually booting the image.
I have verified these USB flash drives *do* boot on another computer that has BIOS USB boot support.
Nevermind. I was able to get my motherboard to boot straight from USB by flashing it with newer BIOS from a different motherboard. Yes, a different make/model motherboard, and it didn't brick it.