HDD Boot from ipxe.efi loaded via network

[simon]

So, we have more and more machines running EFI now, and on some of them we need to dualboot a local OS. With PCBIOS this was dead simple, as sanboot worked flawlessly on 99% of machines, and for that 1% where it didn't, we chained to syslinux6 with chain.c32.

With EFI, things aren't that great. As far as I can tell, you should be able to just set the boot order to Netboot first, HDD second, and if you want to HDD boot from ipxe, you exit 1 to signal that the boot method failed and the EFI firmware should continue with the next item in the boot order.
Our sample size of system models where we use EFI is only 6, but sadly, only one of them works with the method described above. Others just freeze, show "no operating system found" or do other silly things. I picked up somewhere that exit 0 instead might do the trick, but to no avail; funnily enough, on an HP Elitedesk 800 G5, exit 1 shows a "Startup Menu" with a bunch of options, while exit 0 either re-starts network boot immediately, or shows a "Boot Menu" where I see exactly the boot order I configured in the EFI setup and can choose from them.

[Update: the HP Elitedesk 800 G5 works with "exit 1", but only if we use snponly.efi instead of ipxe.efi]

Did anyone figure out a sane solution to this, or are there at least concepts of how to improve the situation? Can anything be done on iPXE's side? Can we load another bootloader via network that can access the ESP and load the default entry?