clod

there's a small pxe bootstrap @ http://www.clod.pwp.blueyonder.co.uk/nbp.zip that you can use to access a secondary pxe nbp with a simple password. run the binary and save the patched nbp.0, boot this pxe (rename it to suit if need be). if left-alt and shift are held down at boot, it asks for a pass then boots secondary nbp, otherwise the default nbp is loaded. it was written in a hurry nearly 10 years ago for use in a school, and not touched since. (worked ok with zfd 4-7)