Debian Installer preseeding from USB Stick

I had the idea of using a debian mini iso usb stick and some preseeding to quickly install some machines. I have some 'late_script' and 'preseed.cfg' magic already for network booting but using a USB stick is a little more tricky. What documentation does not tell you is that when using a 'file://' instead of a 'http://' url on the kernel command line the d-i does not append stuff to the url as it does using http where it extends by 'd-i/'.

Now i have a usb stick (cat'ted the boot.img to the usb stuck and put an mini.iso in the resulting filesystem) with a syslinux.cfg which installs a simple system with most of the packages from the usb stick itself.

LABEL def
        kernel linux
        append vga=normal initrd=initrd.gz netcfg/dhcp_timeout=90
        priority=critical language=en keymap=de country=DE
        locale=en_US.UTF-8 auto
        url=file:///hd-media/d-i/jessie/./preseed.cfg
        classes=vc/sys/simplevirt

I am heavily using Phil Hands d-i extensions for building classes. This is very recommended. I am maintaining tons of workarounds for broken packages in there since the lenny release.