I was thinking to mount the upper strap (I will be using a Y harness) to the upper standard backseat seat belt hole in the C-pillar. The belt would sit a bit crooked to the outside, but on the other hand the angle of the upper strap would not be to sharp. I've seen recomendations to have this strap not going down in an angle steeper than 10°.
I personally do not think this would be a good idea. In the remote chance of a rollover, having that upper strap mounted way off to the left would tend to jerk your upper body that way as well. Not a good idea when your lower body is square in the seat. The idea is to keep your entire body firmly in the seat during a problem. Go to Simpson Safety's web site for mounting specs.