Starting with the release of MAU 3.3 in December 2015, MAU understands 4 channels:
The Internal channel will be builds 2 weeks prior to production. The Internal channel will remain undocumented with no GUI but you can set it via a plist pref 'ChannelName'
defaults write com.microsoft.autoupdate2 ChannelName -string 'Internal'
The External channel will be a week prior to production. In MAU 3.4 which will release in January, there will be a checkbox to opt into the Insiders program which will set the channel to External.
defaults write com.microsoft.autoupdate2 ChannelName -string 'External'
To disable the checkbox and prevent end users from subscribing to the Office Insider program:
defaults write com.microsoft.autoupdate2 DisableInsiderCheckbox -bool TRUE
(self explanitory, more info to come later about release cycle?)
Setting the ChannelName to 'Custom' will enable another pref called 'ManifestServer' where you can point to your own server.