Thank you! RetroArch does have a general "Gun D-pad" set of inputs. I can indeed assign the D-pad on my gun to that and steer the car now. I noticed that the controller can also now steer in lightgun mode, which is good if someone just wants driving duty while 1 or 2 other players shoot. 😃
The standalone controller mode seems unaffected by these changes, so all seems good now.