[syslinux] Equivalent of pxechain.com for (USB key based) syslinux

[Alain Knaff]

Hello list,

I am trying to prepare a USB disk with syslinux that is meant to be able
to offer a choice among many different distributions. The goal is that I
should be able to dump each distribution's CD contents into a separate
subdirectory, and have a master menu that hands off to each
distribution's subdirectory.

A while back, when faced with a similar task on a PXE server, I came
across pxechain.com which elegantly solves this issue. With pxechain.com
I can say the following in my master pxelinux.cfg/default:

default menu.c32
prompt 0
menu title LGL PXE boot menu
timeout 100

LABEL KubuntuNet
        menu Kubuntu network install
        kernel pxechain.com
        append ::kubuntu904/netboot/pxelinux.0

LABEL udpcast
        menu label Udpcast
        kernel pxechain.com
        append ::udpcast/pxelinux.0

This correctly hands off to the (unmodified) pxelinux.cfg/default's in
the distribution's subdirectories. All KERNELs and initrds now use the
subdirectory (kubuntu904, udpcast) as their current directory, rather
than the original root directory.

However, how can I achieve the same thing with syslinux (for USB keys)?
There is MENU INCLUDE, but it doesn't change the current directory, so
the submenu file must be edited to prepend the subdirectory to every
kernel, initrd, display, function key, further submenues, ...

Thanks,

Alain