I don't know the specifics of the Net6501, but can't you do a foreign debootstrap where you let debootstrap copy all files on a machine that is already installe

I don't know the specifics of the Net6501, but can't you do a foreign debootstrap where you let debootstrap copy all files on a machine that is already installed, then boot it up on the Soekris itself and let it do the rest of its install work over the serial connection?