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I don't think that a new EMS Actuator to override the Maximum Flow Rate input field of the Fan:ZoneExhaust object is necessary. The maximum flow rate is a user input that cannot be autosized, so you already have control over what that value is. The zone exhaust fan can then have its flow rate change over a simulation according to the Flow Fraction Schedule Name input field. If you leave this schedule name input field blank the exhaust fan will always operate at maximum flow (assuming no other input fields are set to turn it off).

If you want to change the zone exhaust fan flow rate over the simulation, then you want to assign a schedule to the Flow Fraction Schedule Name input field. This should be a new schedule that isn't referenced by anything else in the model. You can then use the EMS Actuator for "Schedule Value" to set the fraction of maximum flow rate of the zone exhaust fan. This post has an example of using the schedule value actuator as an HVAC system on/off setting. Note that this schedule value actuator can only be applied to four schedule object types: Schedule:Year, Schedule:Compact, Schedule:File, and Schedule:Constant.

The excerpt from the I-O Ref that you included in your comment is referencing the EnergyPlus Github site where the source code is hosted. Here is where you can "use the regular system(s) for user support to request it be added to the program" by making a new issue for the EnergyPlus development team to review. This Unmet Hours forum is only used to answer questions about using building energy modeling simulation tools, so this isn't the appropriate place to make such a request.