Comment 11 for bug 1190256

for me ubuntu-14.10-desktop-amd64 boot with multibootusb on a USB stick fails with:
---
SYSLINUX 6.02 EDD 2013-10-13 Copyright (C) 1994-2013
Undef symbol FAIL: __syslinux_shuffler_size
Failed to load libcom32.c32
Failed to load COM32 file gfxboot.c32
boot:
---
both of these above mentioned files are present in:
"F:\multibootusb\ubuntu-14.10-desktop-amd64\isolinux\gfxboot.c32"
"F:\multibootusb\ubuntu-14.10-desktop-amd64\isolinux\libcom32.c32"
and are exactly the same size as within the SquashFS archive:
F:\multibootusb\ubuntu-14.10-desktop-amd64\casper\filesystem.squashfs\usr\lib\SYSLINUX\modules\bios\

libutil.c32 file also matches perfectly, however menu.c32 is missing. Will try booting it after copying menu.c32 from squashfs to isolinux and will report back.

I suspect this is fixed (well, changed at least) upstream;
http://www.syslinux.org/wiki/index.php/Syslinux_6_Changelog says:
Changes in 6.03:
...
core, BIOS: Move __syslinux_shuffler_size to assembly. Actually get the shuffler size sanely. Fix booting recent Syslinux versions on certain hardware and on certain virtualization software (e.g. QEMU 0.11.1, VirtualBox 4.1.x).
...

Changes in 6.02
BUG Warning: Using Syslinux 6.02 on BTRFS volumes corrupts the superblock. Use 6.03 or newer .

Interestingly booting the same source iso directly in the oracle virtualbox works fine.
Also earlier ubuntu-14.04.1-desktop-amd64 boots with no problems from the same USB stick and same multibootusb on the same computer.