This TileMill plugin exposes syncAccount and syncAccessToken in the TileMill Application Setting window.
The plugin should be available from the Plugins panel in TileMill. You can also install it manually by cloning this repository into your TileMill plugins directory.
Note: This plugin is not tested to work with other plugins.
Enable the plugin
Go to Application Settings and you will see the new sync information.
use this information to create your own TileMill batch upload scripts (see http://bit.ly/AutomateTileMill for an example)