There's a quick and easy method of installing Ubuntu (or any derivative) from a pendrive: isohybrid.
It's a tool which is part of the syslinux package.
just install it, then type
isohybrid FILENAME.ISO
where FILENAME.ISO is your iso file. The md5 checksum will change, of course.
Then, you can just write your modified iso file to a pendrive:
sudo su
umount /dev/sdx
where sdX is your pendrive device name. Pay attention!
dd if=FILENAME.ISO of=/dev/sdX
I wonder why the standard ISOs aren't already patched with this. Or maybe they are and I didn't know it?

Hope this helps