I think i got the reason of your problem. I think that you should start the script enable.sh from rEFIt at the end of the process to make it takes into account the syslinux configuration
I used your idea of the HFS+ partition and then use the walkthrhttp://www.backtrack-linux.org/wiki/...Persistent_USB
Now i am able when i boot my macbook pro 5.5 to see the USB key, select rEFIt with will display 2 entries : HD and Syslinux. Then i select Syslinux and it boots perfect
Have fun !