Can the stock seats, which I love, support a 5 or 4 point harness system without modification? I guess they could be fed through the rotory-triangle opening in the upper center, but is that the only way?

 

Not safely

 

Why not? Will the belt just crush down through the seat?

This might be better in the competition forum.

 

Putting both belts through the center hole doesn't leave enough room for your neck. Those belts need to secure above the shoulders, not behind your head. A custom 3 point harness would work if designed properly, but stuffing belts from a 4 or 5 point harness through that hole is asking for trouble.

 

There is at least one other thread in here somewhere on this - might be in the Racing\competition forum.

The accepted answer is what asmoran has said don't do it, get aftermarket seats and a roll bar to attach the harness to if you are going to do serious track time.