build emdebian wheezy for a custom target

I'm embarking on a project to build emdebian wheezy for a custom embedded
target.  The build has to be cross-compiled using a custom toolchain for a
custom architecture.  The build process will therefore requiring download
the source code for all software, cross-compiling with the custom toolchain,
and then packaging it up into a root filesystem.

Is this something that multistrap can handle from start to finish, or do I
first have to download all the source packages, port them to my custom
architecture, repackage them, and then hand them over to multistrap?  Would
someone be able to point me to any How-To's or examples of this type of
build, especially any that include the necessary config files and/or shell
scripts?

Thank you much.