duhh-When the camber link and A-arm are parallel and parallel to the ground as is often the case on off-road trucks the instant center is at infinity. When you draw a line from the contact patch to meet the instant center it is parralel to the other two lines. The roll center is indeed at the ground (where the two lines from the tires cross the center line) which is a good place for it to be.
Yes the instant center can be outside the chassis. On our symmetrical touring cars (and the off-road truck) the roll centers will always be on center line of the car. It moves just a little up and out with roll but not much.
My experience with a touring car and Carrol Smiths experience with full size race cars in Tune to Win
is that generally an improvement caused by changes to the suspension link positions is usually due to an improvement in the roll center. I find that the touring car is relatively insensitive to camber changes. So when you move a link pay attention to what it does to the roll center. Although crazy roll centers have been used on RC cars as well as full size cars I think that you will have the best luck with rolls centers near the ground or 1/4 inch high or so on a touring car.
When you put the links like you suggest you get a roll center very near the center of gravity. This increases the roll stiffness to an amount so high that traction is reduced. It also increases suspension jacking forces that jack the suspesnion up so high that you cannot use it on high traction surfaces. You can get a snap roll or traction roll. So forget the very high roll center. The optimum is somewhere in between the ground and 1/4- 3/8 inch or so.
Now if you care to know where your roll center is exactly I have a Microsoft Excell spreadsheet that with a few measurements you can calculate the roll center. Usually this is not neccesary, but it has taught me a few things.
Usually one hole up or down on an inner camber link pivot is way too much change. It is better to shim lower A-arms up a little to raise the roll center or shim the outer camber link up a little to raise the roll center. Start low and work your way up after you find the right springs.
to request the roll center calculator.
Here is a link to another thread on this web site with a picture of the output
of the calculator as well as the measurements to be taken, and also some discussion of roll centers.