Imho, something like http://stackoverflow.com would be perfect for
Lilypond. I know it from python programming and it's so convenient to
see if there is a question matching the own problems and then to have a look whats the best answer there.
Stackexchange might be a technical solution. But you find Lilypond tagged questions in different sites of StackExchange: in Stackoverflow as well as In Tex. It would be good to have questions in one database to reduce diversity. I don't know if it would be desirable to create a StackExchange site for Lilypond and what effort it costs.
Helge