I have found a method to assign joystick buttons to controls within the cockpit that can normally only be operated with the mouse. The P40 only has a few.
First for an explanation on how this works see this thread in the P47 Accusim section, its long so thats why im not repeating it!viewtopic.php?f=55&t=19524
For the P51 use the Macros below and create a file as per the P47 explanation. Call it 3P40.MCRO
The 3 will mean that these controls are near the front of the list ( I have used 1,2 and 4 for other planes).
Most have parameters of 0,1. OxyAltSet is 0-100 as is Cabin vent.
I have only experimented a little with the first three commands, the propellor set ones but havent got them working quite right. Try combinations of parameters 0,1 and 2