We have a thread on the topic. Some headway was made, but unfortunately the main guy is MIA and their repo (concept not code) was deleted:
Here is an archived version:
We are at a point where some things need to be automated/scripted. I think concentrating on including I2P by default in the Workstation is the easier and higher yield route.
Main task would be to optimize and create a second TBB capable of connecting to localhost daemons. This would be useful for zeronet and Freenet and potentially many other uses. May also be relevant to out Tor Browser without Tor version for Hardened Debian.
Adding I2P repos by default and fetching binary from there during build time. We may need to make this part an optional build time parameter according to what @Patrick thinks is best.