OK, I KNOW it can be done, but with a little circuit trickery.
What you would need to do is add a small time delay relay circuit to the Joycon, such that when you depress a steering wheel button, and the Joycon detects it, it outputs a signal to switch the relay, which you wire to open the normally closed D+ and D- and hold it open for say 30 seconds to allow the Joycon as being connected.
You would need to find out what the shorting resistor value typically used for the D+ and D-, and add that value of resistance into the N.C. circuit, probably 100-1K ohms would work fine. This is just a safety feature to protect the USB controller on your phone.
So you press a steering wheel button of choice, the relay opens, a few seconds later your Joycon is detected as connected by your phone, you use the buttons you want, like volume, next track, etc...
After the timeout period the relay closes again, and say 100 ohm resistor is effectively shorted across the D+ and D-, and your phone starts charging again.
Too much work? Just wire in a data shorting switch somewhere to toggle between Joycon or Phone Charging.