While deploying Configuration Manager 2012 the team was gathered around a desktop machine configuring the basic settings of our CM12 sites. We needed to configure the network access account and even with 6 people it took us quite a while to figure out how to set the network access account. Have you found it?
Navigate to the Administration node. Open Overview/Site Operations/Sites and right click on the site, choose Software Distribution and select the Network Access Account tab.
It does make sense that the NAA is specified as a site setting rather than a client setting.