Any current unit that doesn't have a built-in floppy drive, will allow for USB booting. The only floppy drives that you can use on these units will be USB. Just enable the feature in the BIOS (USB Legacy Support), but by doing so, you disable USB 2.0 functionality until Legacy Support is disabled.
Interesting; I didn't know that. So for the rare "emergency" boot, you'll need to temporarily disable USB 2. Thanks for the tip.
Now, how about booting from a PC card (PCMCIA) drive? I have an IBM 8GB drive, and thought it would be handy to be able to boot from it if I had to. Do the BIOSes of these HP models I mentioned (zx5000, zt3000) support booting from PCMCIA?