Another option I've done is to connect a toggle switch to 2 of the GPI ins and set one to control the mute of one channel and the other to a mute of a second channel. Since the switch can't activate both GPIs at the same time, toggling the switch back and forth toggles the mutes between he 2 channels.
This is probably the best bet for David's setup, the CL can do 5 GPIs, so can control 5 individual main/BU changeovers. Using scenes can get pretty complicated if you have multiple scenes and multiple mic BUs, although I think you can do more than 5 with scenes. Using scenes for more than 1 requires being able to focus a scene to just a single patch without recalling the whole patch.
Mac
