There is not a simple software way of doing this, but there is a way. Wire the output of the variable through a relay before it goes to the fan. (L1 goes to one VAR terminal, the second VAR terminal goes to the relay, and then the second relay terminal goes to the fan). Configure the relay as a heating duty cycle with a setpoint at the idle range of the var fan, and a secondary modifier forcing it on above the idle range of the var fan. You will also need to add a secondary modifier to the var fan that forces it to your desired speed for duty cycle min vent when the temperature is below the idle range setpoint.

This will mean that the relay acts as a disconnect relay for the variable signal. Above the idle range setpoint it is forced on, allowing the fan to vary as it should. Below the idle range setpoint the fan will be locked into whatever you want the min vent idle speed to be, and then the relay will cycle on and off according to the settings for a duty cycle heater.