It is currently quite a lot of work, on source packages with lots of
binary packages, to tell which packages were built for which arch.

A list of archs for which the package is not available (or a list of
archs for which it _is_ available, when that number is less than the
number of archs relevant to the source package) could be appended to
each relevant binary package.

Eg: currently it looks like gcc-4.0 does not build gij-4.0 on at least
mips and mipsel.