I'm in the middle of modernizing my Eagle and I was going to look into something of this nature.
The pillar could be cut into and modified by welding a nut on the inside, drilling a hole to it and welding back in but it would still be relatively weak compared to reinforced metal. It would probably hold up though.
Another thought is to look for a seat belt that loops over the back of the seat and connects to the floor behind it. I have seen it in an SUV but I can't remember which one. It struck me as odd, but a good way to retrofit more of a modern belt to the Eagle.