From Abovan:

Would it be possible to have GreenWall intercept the hidden guild comm channel and broadcast them to all of the guild members using GreenWall? This would allow other addons that send information to the guild, such as Gatherer to transmit their data to the whole guild and not just their subguild.

This can't be done transparently with the current API. The best alternative is to have GreenWall-aware addons use the API.