I would as well - and I'd also advocate
Bnlcmbcar to do some searching some of the DIY audio forums that have home theater sections.
From the diagrams that you posted, I'd be concerned as you are (especially in a car) that you'd narrow the image, because you are pulling some of the L and R content into the center - which, most troublingly - brings some of the L content to your R side, if you are in the driver's seat. Conversely, for a passenger, brings some of the R content to the L of them. Eek.
What I'd like to see in a center channel for a car, would be "(L+R)-(L-R)". That way, it's only bringing the content from both speakers to the center, and even if there's a little content that's on both channels but simply louder on one than the other, it still mitigates that by attenuating that sound at the center channel. I'd believe that would at least
help keep the stage width - hopefully as wide as with just a stereo pair.
But I think really, in a car, the KISS rule applies... there's already glass, and absorbant upholstery, and plastic, all pointing different directions - you could have ONE speaker in a car and end up with a nightmare of multiple pathlength distances, direct and reflected (with each of those having a 180 degree shift - plus pathlength difference offset!) creating anything but a flat response as it arrives at your ears.
So I subscribe to the "the fewer speakers the better" theory for car audio. There's exceptions - for example, three way components where you actually aim the mid and tweeter - can provide better imaging. But my default recommendation is simplicity over complexity, for those pathlength reasons.
I'm really interested in this myself, but in no way for creating a center channel (IMO, there's already enough direct and reflected pathlength sounds wreaking havoc on image-killing phase interactions as they all ultimately arrive at the listening position)...
I want to make a "L-(L+R)" channel and a "R-(L+R)" channel to add some rear fill (something I otherwise also don't believe in, for those same image-killing phase interaction reasons) plus some additional delay, so I'm going to be researching this soon myself. My DSP will only help with delay and passband. I'd be interested if you find any good threads.