Using apt-get with Architecture != uname -m

[Marc Singer <elf@buici.com>]

I'm using apt-get to fetch packages along with their dependencies into
a local archive in order to create a filesystem for an embedded
system.  It works well when the target architecture the is same as the
host architecture.  However,

  apt-get -o DIR=to/archive -o Dir::State::status=status \
    -o APT::Architecture=arm -d -u --force-yes --reinstall -y install \
    ash base-files base-passwd busybox [...]

returns an error 25600.  I'd expect that the Architecture switch would
allow me to download the packages.  Is there some other purpose for
the switch.  Should this not work?