I've done it, prototype only, but I had a box which could do RA menu adjustments with single button presses, LED indicators for the handbrake and headlights and I designed it to integrate with a standalone H-shifter but I never got round to actually making that because I bought a G27 instead. It was also a bit hacky, you had to know how many gears your car had and never accidentally mis-shift into, say, 6th when you're in a 5-speed otherwise it would get out of sync. I could make it up again if anyone's interested.
Edit: the Cthulu board is a good alternative but you don't get a PS button nor analogue sensitivity, and I'm sure most people have a worn out controller or two. I'm pretty sure you could pick up a cracked controller on EBay for cheaper than a Cthulu too, and yeah, it's more work soldering to the board than using screw terminals, but it's down to your skill and what you want I guess.