Stupid question: what does Omniref do that can't be done with source code comments? Comments are readable by anyone in any editor and do not require external tools. They are co-located to the relevant lines of code by being part of the source, and are as portable as the code itself. My first impression is that it is a solution seeking a problem.
The problem seems to be post-commit code review or something like that. I don't know if they did, but I hope they used git-notes which has been in git core for a while now.
