Single-zone VAV heat pump in OpenStudio

I see a few options in EnergyPlus to model a single-zone heat pump with variable air volume, but I haven't found a way to do it in OpenStudio. In EnergyPlus the AirloopHVAC:UnitarySystem object has an option for control type SingleZoneVAV. Or multiple stages of airflow can be represented with the "Design Specification Multispeed" Object Type. But neither of these appears to be available as an option in the OpenStudio interface.

Before I go outside OpenStudio, I wanted to see if anyone has a successful method for representing this system type within OpenStudio. Variable flow has been required by code for some time now for systems larger than 5 tons.

First, you're right that OpenStudio's OS:AirLoopHVAC:UnitarySystem is missing the control type option SingleZoneVAV which was added in E+ 8.7.0. I opened a github issue: #3280. Until it gets added to the OS App, a dead simple EnergyPlus measure could be used to switch the control point of each unitary system. Something like this:

worskpace.getObjectsByType("AirLoopHVAC:UnitarySystem".to_IddObjectType).each do |idf_unitary|
  idf_unitary.setString(1, "SingleZoneVAV")
end

Second, multispeed DX coils are wrapped in OpenStudio SDK, there just isn't any example in hvac_library.osm so you can't just drag one from the Library to your model in OS App.

But you can find several examples of demonstration of these objects using the Ruby API if you go to the OpenStudio-resources repository, in model/simulationtests. You want to look at these:

autosize_hvac.rb
multi_stage.rb
unitary_system_performance_multispeed.rb
unitary_system.rb

If you aren't that comfortable with the Ruby bindings to create all of your systems, you could just use the ruby bindings to modify your local hvac_library.osm and add one example of your system that you could then drag and drop in OS App. (I don't recommend doing this, but you could also just manually add all objects required for a unitary system with the inner components that you need in text mode, just mind that the 'handle' must be unique UUIDs)