Could have been helpful to share what type of AC system you plan on using, but either way I think you can do the following:

• Set the zone Thermostat's cooling temperature to 26°C
• Use an AvailabilityManager:HighTemperatureTurnOn to sense the Outdoor Air Temperature with a temperature setpoint of 29°C.

This would look like the following, making sure you replace the 'Sensor Node' by a valid Outside Air node

AvailabilityManager:HighTemperatureTurnOn,
  AC HighTemp Turn On Availability Manager,   !- Name
  Outside Air Inlet Node,   !- Sensor Node
  29.0;   !- Temperature {C}

• Package the AvailabilityManager:HighTemperatureTurnOn into an AvailabilityManagerAssignmentList

• Use this assignment list into your zone air conditioner. For example, if you use ZoneHVAC:PackagedTerminalAirConditioner, you would put it into the Availability Manager List Name