Problem:
As others may have also observed, some car seat manuals (see attached photo) explicitly state that for front-facing position "you must move the child restraint to another seating location" IF "vehicle [shoulder] belts in center of door or side panel" - the latter describing exactly the 3rd gen rear shoulder belts. The reason for this, I have observed, is that because the shoulder and lap belt thread thru the seat in the same hold, the lap belt wants to pull down while the shoulder belt wants to pull up, resulting in the car seat wanting to twist no matter how tightly it is secured. Keep in mind that while your car seat may be tight when the vehicle is not moving, in a crash or rollover, that seat is going to want to go every which way.
Possible Solution:
Long story short, instead of trying to find some way of making UAS (Universal Anchorage System) anchors, or making the shoulder belt work, one might easily be able to replaced the shoulder belt with a satisfactory lap belt. The 4runner has 2 bolts for the lap belt, so I would think it would be easy to find a new or used lap-only belt to bolt in on top or replacing the door-side belt attachment point (which is just a standard bolt). Will investigate...
The more I try to find a solution the more I realize how important it is to secure your child seat properly. ALot of research goes into these and the whole reason you're buying and installing them is for a worst-case scenario, so despite what the wifey might say "good enough" is quite possibly not enough. I used to fasten the belts in the car-seat a bit loose because I was worried about my kid's comfort, but now I realize that their seat belts need to be as tight as possible without squishing the kid... which probably is isn't the most comfortable but they need to be restrained, even if it means they can't reach their water bottle, toys or books.