Update: Based on your entry, I found the article "Grub chainloads syslinux and back", which basically says "patch grub":
Chainloading syslinux from GRUB seems not to be
possible without a patch. I don't know about GRUB2.
This could be useful for multi-booting (syslinux-based) partitions.
Chainloading syslinux from GRUB4dos (on different partitions) is
possible. I managed it through the mapping, explained (again) here.
It solves some problems in multi-booting usb devices.
The patch page mentions a similar patch was posted to grub-devel for Grub v2; this post may be that patch. Or this message, which points to this bug report (dated Nov.2009, so it's very recent) for a Grub v2 patch.
To use one of the patches, you'd need to download the Grub source code for your preferred Grub version, check that it doesn't include the patch already, try applying the patch, and recompile and reinstall your new Grub.
Original answer:
I'd try testing a simple Grub entry first. This is a standard Grub entry for the Memtest86+ utility on a Debian system:
title Debian, memtest86+
root (hd0,0)
kernel /boot/memtest86+.bin
Just try tweaking those with your values (change the root partition or kernel path as necessary):
title Ubuntu LiveUSB SysLinux boot
root (hd0,0)
kernel /syslinux/isolinux.bin
I don't think you'd need the chainloader+ command for this task.
Best Answer
Try this tool, you copy it to your /boot folder and add an entry in the menu.lst of grub, it will list every possible boot device available including USB even if your mobo does not support it.
Read the instructions and be careful not to mess your boot sector.
Jim