Currently in-game, there is no way to assign a dedicated keyboard button/mouse axis to the inputcontroller, except by using an existing input option and binding it with a dedicated key/mouse axis.
I would suggest you add the option to assign any key (from A-Z), every conventional mouse movement.
To ensure that there's also something here for mobile users to profit from, you could add an input option for dragging on the screen and device XYZ rotation.
To make sure this doesn't interfere with any normal in-game controls (pan to look around, etc.), you could force it to only work when a certain activation group has been activated.
Last but not least, it could be in this style: DirectInput.MouseX
Please review this because it will increase the creativity of our crafts.
@WNP78 sorry for the ping, but would this be hard to implement?