AVG version used: 10.0.1204
It looks to be as simple as accessing the 'Update from directory' item located in the 'Tool' menu. Scheduled updates should be controlled through the normal Update Scheduler (Tools - Advanced settings - Schedules - Update schedules).
I can't find an option to controlling the updating behavior in the command line documentation. You might need to use a different Anti-Virus product that is more flexible.
Are you sure you can't set it through the normal update scheduler? I have the option to do so in 10.0.1204. I know it's not scriptable, but it looks like it is the best you are going to get.