If you are on Windows, that's really trivial: use Joystick Gremlin, and you can map anything to anything, mouse buttons/wheel included.
I'm using a Virpil Constellation Alpha Stick, and it has a wheel kind of dial on the stick. I map the up down rotation to mouse wheel up/down, and the wheel press to mouse left. I them mapped the 'black' right button to mouse button 2. Additionally, I mapped the right rotary dial on my Throttle (Virpil Mongoos 2) to also be used as up/down scroll wheel. so I can use either hand. Total time to set up gremlin: 1 Minute
On the Mac, you may use Joystick Mapper or similar. Caveat Emptor: I have no experience with Joystick Mapper on the Mac (although I'm alsoe an avid Mac head).
To only problem that now remains is making the mouse pointer move to where the HMD is looking. This can only be done from within the app itself.