Is there some documentation on the packaging steps (redistribution layer, wirebonding , flip-chip, interposers, bga) and any possibilities for ChipIgnite users to get unpackaged wafers, that could get additional process steps from a different fab? I have a user that is interested in that...

There is a fair amount of documentation in the caravel repository under (I think) docs/ (it got moved, and converted to Sphinx over my protestations). ChipIgnite projects will alternate between WLCSP and wirebonded runs. We have done some handling of custom work with additional process steps. It rather severely hampers our ability to streamline the shuttle run process to keep it fast and reasonably cheap. You would need to contact Jeff DiCorpo about what our requirements are for that kind of special handling. For the WLCSP, there is a bump bond layout in the caravel repository. It is not a BGA; it is WLCSP, and the solder bumps are manufactured directly on the top of the wafer. So there is a redistribution layer but no interposer. The bump bond pinout and example circuit board footprint are included in the documentation.