I actually solved this and it works! I found a cheat code called: live-media-path=path/to/casper/folder along with ignore_uuid. So the modified working code:
DEFAULT Backtrack PROMPT 0 allowoptions 0 TIMEOUT 100 UI vesamenu.c32 MENU TITLE Disqus: The Ultimate Disk MENU BACKGROUND anon1.png MENU COLOR border 30;44 #40ffffff #a0000000 std MENU COLOR title 1;36;44 #9033ccff #a0000000 std MENU COLOR sel 7;37;40 #e0ffffff #20ffffff all MENU COLOR unsel 37;44 #50ffffff #a0000000 std MENU COLOR help 37;40 #c0ffffff #a0000000 std MENU COLOR timeout_msg 37;40 #80ffffff #00000000 std MENU COLOR timeout 1;37;40 #c0ffffff #00000000 std MENU COLOR msg07 37;40 #90ffffff #a0000000 std MENU COLOR tabmsg 31;40 #30ffffff #00000000 std MENU WIDTH 80 MENU MARGIN 10 MENU ROWS 5 MENU VSHIFT 10 MENU TIMEOUTROW 13 MENU TABMSGROW 18 MENU CMDLINEROW 11 MENU HELPMSGROW 16 MENU HELPMSGENDROW 29 LABEL Backtrack MENU LABEL Backtrack kernel /syslinux/iso/backtrack/casper/vmlinuz append initrd=/syslinux/iso/backtrack/casper/initrdf.gz live-media-path=/syslinux/iso/backtrack/casper ignore_uuid boot=casper quiet splash -- LABEL Ubuntu MENU LABEL Ubuntu kernel /syslinux/iso/ubuntu/casper/vmlinuz append initrd=/syslinux/iso/ubuntu/casper/initrd.lz live-media-path=/syslinux/iso/ubuntu/casper ignore_uuid boot=casper quiet splash -- LABEL Mantra-OS MENU LABEL Mantra-OS kernel /syslinux/iso/mantra-os/casper/vmlinuz append initrd=/syslinux/iso/mantra-os/casper/initrd.lz live-media-path=/syslinux/iso/mantra-os/casper ignore_uuid boot=casper quiet splash -- LABEL Reboot MENU LABEL Reboot COM32 reboot.c32 LABEL Power Off MENU LABEL Power Off COMBOOT poweroff.com
Many thanks!
P.S: For other distros you may look for syslinux configuration files in Yumi's repository at GitHub: https://github.com/bwnickle/YUMI/blob/master/src/menu/