I haven't tried this with Mandrake (though I've done it with other distros...)
You would need to unpack the boot.iso to your USB device then use the syslinux to set up the boot sectors.
The data is already there in the boot.iso file so everything should already be in place.
You the run syslinux making sure that it finds the syslinux.cfg file (which may need to be specified...) against the USB device.
The Puppy Linux page illustrates how this is done.
It would be interesting indeed to have a flash bootable Linux install "pen" or drive available...