The MCPs are currently hosted as gdocs; current frontend is http://bit.ly/mcp-index. There are a lot of tools that can convert gdocs to markdown, preserving revision history. MCP-1 is the draft doc for migrating to github.
A major issue with migrating to github is familiarization; most NoMers to date seem much more comfortable with gdocs.
Another major issue is the live collaboration features of gdocs -- critical during meetings, breakouts, and conference calls. Etherpad would be able to supply that feature, but we'd want to use markdown and git as the backend, which would take some hacking on etherpad iirc.
A possible solution might be using etherpad as the master, with mirroring to github or gitea.