Implementing @Archmage tips on actuating the Mass Flow Rate for the AirTerminal:SingleDuct:Uncontrolled and sensing the zone output, it was still not working.

I actually started investigating whether it was some kind of sizing problem. Here's what I did to make it work:

• Removing for the cavity the Controller:MechanicalVentilation. This had a problem because I never defined a space type for my Cavity (which is like a plenum...) and therefore does NOT have a DesignSpecification:OutdoorAir object associated with it
• Removing the sizing:zone (my zone is a plenum effectively), and,
• Hardsizing the flow rates of the AirLoopHVAC, Controller:OutdoorAir and the ATU. Not sure if I need to hardsize all of that...

I think the real problem was the Controller:MechanicalVentilation one and it is probably deeply related to this issue: Controller:OutdoorAir Field: Minimum Fraction of Outdoor Air Schedule Name

For the record, actuating the Fan mass flow rate instead of AirTerminal:SingleDuct:Uncontrolled mass flow rate does NOT work.