It's possible if this debian-installer thing can be convinced to obtain its files from the drive instead of the network somehow. installing a bootloader like grub on a USB flash drive is
possible but sometimes tricky. Once you can get grub going, it's just a matter of experimentation until you find the config that'll let it boot the files you want.
How these machines are booting is they download /tftproot/pxelinux.0 and run it, which in turn downloads its config file, /tftproot/pxelinux.cfg/default to find out exactly what it's booting with what parameters, then finally, downloads and executes the kernel and initrd its config files told it to. Viewing the contents of /tftproot/pxelinux.cfg/default should be enlightening.