Sorry i have not enough time to work on this problem. But today i've tried with the last release of syslinux (http://www.kernel.org/pub/linux/utils/b ... x-3.84.zip)
i've add the file memdisk in the folder and i've changed the name : /boot/syslinux/memdisk-new.
Then i try in my virtual machine (updated) Sun Virtualbox 3.1.2 r56127, and it works ....
Between your message and today fortunately the new release of syslinux is out (3.84) and it seems to solve my problems with custom images.
By the way i'm using these old software to create new partitions not to resize them. i'm using directly software included (by Microsoft) in Windows Vista / Windows 7 for shrinking space.
Thanks again for your help.

Joe
PS : with the new release of syslinux (3.84) i don't have to add commands to the APPEND line, it seems to work out of the box.