Entirely disk based?

[robinsmidsrod]

You need to compile iPXE with an embedded image. Look for documentation on the EMBED_IMAGE parameter on how to embed a customized ipxe script.

Your embedded ipxe script will have to use the prompt command to ask the user to press some key within a configurable timeout. If they do nothing, the script can use the exit command to let the normal BIOS boot order take over.

All this of course relies on your BIOS being proper and honoring the exit command to continue the boot order properly.

You could avoid using an embedded script, but then it would only work when the computer is in the "home network" where you have DHCP setup properly (then again, this might be the only place you actually need this, unless you have laptops needing this kind of setup).