As mentioned a while ago, I was looking for a firmware packager that would allow for "user uploads a signed bundle" style upgrades for hardware devices.  Esky is similar (even a similar on-disk layout), but focused on cross-platform and distutils-based "application" setup.  Fussy is intended for "core of a system" setups, where the entire user interface and functionality of a custom piece of hardware is delivered through the "firmware" package.

The package is up on PyPI, LaunchPad and ReadTheDocs now (though the docs are still being built on RTD). There's still lots to be done, but the basic outline is there.