Okay - using a PS1/PS2 to USB adapter (a cheap, POS one) it functioned out of the box, as-is on all but the analogue BUTTONS (not the pivot, that worked for some reason)

L1, [] and X did not work, though. It's a ROYAL PITA to use in-game with HD, because you cannot access the PS menu, or even navigate ANY menu in game. In order to use it, you need to unplug the adapter, turn on a PS3 controller, select your ship, track, and start the game. Then once the game is about to start (during the count down) you need to set the PS3 controller to #2, and then power it down. Immediately after that, you plug in the USB/NeGcon adapter - and you can play providing you can live without your left airbrake, thrust and fire buttons. FWIW, I did try it on Zone mode - and it worked pretty well - except that movement was EXTREME to register anything at all in-game. (I chalk this up to no driver support - so default value is minimal response)

The PS3 controllers buttons (all of them except for Start, Select and PS button) are ALL analogue (pressure-sensitive) - they change resistance as more pressure is applied. It's no different than the older membrane-type switches - except they were looking for digital signal overall - all that was done to change these membrane-type switches to "analogue" was to round the contact pads off - so that light pressure results in less contact area across the forks - creating a higher resistance. This higher resistance is "seen" as an analogue value. Press harder on the button, and more contact area is applied - lowering the resistance - which is "seen" as a stronger analogue value.

Knowing this - and the fact that the buttons in the NeGcon are membrane-type (except for the L1, [] and X) - they should be directly compatible. (Dpad, R1, T, O)

As for the analogue sticks on the PS3 controller - those are 2 axis potentiometers on a spring pivot - you have an X pot and a Y pot - as you move around, the resistance changes relative to angular position - which is "seen" as an analogue signal.

The NeGcon used this type of method long before the PS controller ever had sticks - the Twist mechanism, L1, [] and X all have a potentiometer attached. The buttons use a spring and rail - as you depress them, a cog on the pot registers the movement. The twist mechanism has a cog coupled to the pivoting axis of the controller. The twist mechanism I'm going to simply couple into the X-axis of the left analogue stick of the PS3 controller.

As for the NeGcon analogue buttons - those I'm still deciding how to tackle. I'm going to power up my PS3 controller and measure the values seen across L1, [] and X at "idle" and at full-pressure. I'm going to compare those values to the potentiometers in the same locations on the NeGcon. if they're ballpark, I'll either shunt or series them with fixed resisters to get the value exact. My other option could be to replace the pots with pots of a better match, OR to replace the pots in those locations with membrane-type switches.