Im thinking of breaking down a controller and making a dashboard to fit nex to my wheel for things like this.
I recently sacrificed a Sixaxis to replace the broken case on my DS3 (I cracked it during a, er, slightly over-emotional defeat in the MW2 era), so I'm considering doing this, except as I have a DFP which has just enough buttons, I'd quite like to investigate the possibility of mounting it on the pedal box to add a reverse gear that I don't need to operate with my hands; It's difficult trying to hold triangle and turn the wheel, in fact it's difficult to even find triangle when the wheel is anything but straight... I think that was the biggest thing that genuinely bugged me about GT5, that reverse was still not an individual gear that you could select through the usual method.
I have no problem using the menu while driving with the DS3, I mean I only really ever use it to adjust the torque distribution and brake bias, I rarely touch ABS, ASM or TCS though and I don't normally keep fiddling with them in a race so I just get to the start/finish straight and adjust it while I can get away with not steering.
I use the triggers to accelerate/brake, right stick to shift (down for shift up, up for shift down, like a sequential), X to bring up the menu and the D-pad to adjust. It would be nice if they'd let us map the D-pad to do other things when the menu isn't up, but I've found with L3, R3, square, L1 and R1, they're useful for things like the horn, headlights, ghost, wipers and all that.
The DFP is the same, sort of, I mean it's easier to adjust and steer with the wheel, but I can't concentrate on both at once anyway.
Actually, the more I think about it, the more I wonder why PD didn't adjust the default controller layout for the PS3. I mean, why use the analogue triggers for a digital action (gears) and X and square for analogue ones? Yes, I know the buttons are analogue, but the triggers are far easier to use in an analogous fashion. It made sense up to GT4, but times have changed!