Well Done!

I'm still stuck on "geometry" as one of the keys to bootability, though. I get maximum bootability by formatting (and creating the partition) with the HP utility even though I then install grub in the mbr - which overwrites the boot code but not the part table. Touching the part table (other than setting the active flag) with Linux fdisk limits bootability on some of my test boxen. (Pretty sure about this - I need to re-test, though with Linux fdisk in expert mode).

I've also tried putting grub in the partition's boot sector and using a box-stock XP mbr w/the HP part table. That seems to boot as well as the HP + grub mbr approach: machines that reject fob formatted w/Linux will still boot this combo.

Still, I'd be curious to know if any other failures happen, when trying to boot from a USB thumbdrive that has been correctly recognized by the BIOS. What BIOS versions are in the computers that worked to boot drives formatted with the HP formatter but didn't work to boot drives formatted with other tools?

That, to me, is by far the hardest problem: getting the BIOS to see the drive, in the first place! If the BIOS can see the USB thumbdrive well enough to set it up as the bootable "hard drive" (the 0x80 device), and run the MBR properly, things just work from there, I have found.
One older Intel MB/BIOS will try to boot my test fob if I use the HP formatter, but hangs solid at "GRUB".

I will try to find the time to experiment more w/geometry and see if my theory has any merit. AND note the BIOSes involved.

Z.