[syslinux] Howto chainload shell.efi (EFI shell) via PXE/syslinux.efi

[JZB]

I have a fully functional syslinux.efi PXE boot environment, and can PXE
boot a linux kernel with no problems.  I run this from a menu system
and my kernel menu's are similar to this:

...
LABEL linux
  KERNEL kernel-5.10.52-linux-gnu-pc
  APPEND root=/dev/ram0
...

I have a "Shell.efi" efi executable shell that I would like to boot into
the same way, so I've added this:

...
LABEL Shell
  KERNEL Shell.efi
...

I have the file in the right location etc., but when I select "Shell"
from the menu, I see a line that says something like

Loading Shell.efi....{bad file number?}

or various other messages (depending on whether I use "KERNEL", or
"COM32", etc), but I can't make out the errors exactly as they go by too
fast.

Anyway, does anybody know the correct way to chainload or boot into a
UEFI Shell executable pulled in via PXE?  I'm running
syslinux-6.04_pre1, and I'm booting into
/usr/share/syslinux/efi64/syslinux.efi as the NBP.

Thanks...