Alright, I figured it out thanks to the help of @Crunchmeister71 in this thread.
You have to bind the Decrease / Increase Autopilot Reference Airspeed keybinds under the Autopilot section!
And as my screenshot illustrates you can bind it to the same buttons as your Decrease / Increase Autopilot Reference VS, which is handy because you will only ever need to do either of those functions when either mode is engaged so it’s fine to reuse some buttons on your HOTAS. 
I can confirm this works (tested in a C172).