OK, yes the passenger seatbelt retractor latching can be disabled. I've just successfully done it on my '97 80 series; and the procedure is fully reversable if needed.
Disclaimer first - The seatbelt retractor is a major safety feature of your Land Cruiser and you could damage the retractor in the process rendering it unsafe; so the safest thing to do is not mess with it. The sole purpose of this mod is to disable the ratcheting mechanism that is intended for securing an infant car seat to the front passenger seat (not that anyone should ever carry an infant in a car seat the front seat of a car that has no mechanism for disabling its airbag). So don't do this mod and if you do I am not responsible for anyone who might attempt it or any damage or injury that might occur should you choose to do so.
OK, on to the details -
1) Remove the front passenger seatbelt retractor as per normal; all of this can be done from the back seat of the truck.
2) Once removed study the colorful side of the retactor mechanism as you pull the belt out of the retractor watching the gears and lever as you pull. See photo below...
The magic of this clever mechanism is in the orange and gray gear and the gray lever. If you slowly pull the seatbelt out of the retractor you will see the orange gear turn counterclockwise and eventually trip the gray lever triggering the ratchet mode just before the end of the seatbelt is reached.
The fix therefore is to simply rotate the orange gear
CLOCKWISE a little such that the cam that flips the gray lever never reaches its destination before running out of seatbelt. It doesn't take much to do this, only about 1/4 turn clockwise will do it; so be careful to not overdo it or it may trigger the ratchet too soon!
3) Getting into the cover is a little tricky too, but it is doable without damaging or altering the cover. For this I pulled the seatbelt out as far as it would go and clamped it there with a large paper clasp. You will see three plastic nubs that secure the cover to the metal retractor frame, these nubs each have a 2mm plastic pin in them that can be pushed thru from the metal side of the retractor toward the plastic cover. I used a small screwdriver with a 1.5mm torx bit on it. You need to push them pretty hard to get them to budge. Once pushed through you can grab them with some needle nose pliers and save them for reassembly.
4) Once the pins are out, you need to carefully and gently pry the plastic cover off of the metal frame, I used my Swiss Army knife can-opener blade for this in order to reduce the chance of cutting myself in the process. NOTE WELL - This cover serves as the gear hub for the small gray gear to note where it is for reassembly. Likewise, the cover is the hub for the gray lever... Be careful, there are a few little parts and a spring or two in there that you do not want to have flying out. Doing it wrong or reassembling this incorrectly may result in the seatbelt not responding correctly in the event of an accident. Go slow. take more pictures as you go, be careful.
5) Once the cover is off, be careful to not mess with the gravity ball or its purple plastic parts.
6) Carefully rotate the orange gear
clockwise about 1/4 turn; then gently and carefully reassemble the cover to onto the frame, making sure to get the small gray gear and gray lever indexed into their associated holes. The cover will click back onto the frame with its original retainers.
7) Before reinserting the 2mm plastic pins, allow the seatbelt to fully retract back into the retractor, then slowly pull out the belt and watch the orange gear to note that it does not go around far enough to trip the gray lever. Assuming you got it right on the first try, congratulate yourself and re-install the 2mm plastic pins (these are a little hard to push back in but they'll go).
8) If for some reason the gray lever still manages to trip and put the retractor into ratchet mode, no worries, just let the belt retract back in, got back to step 4 and pry the cover back off and adjust the orange wheel as necessary again, reassemble and retry.
Good luck with this simple little mod for such an annoying problem, and remember never put a baby car seat in the front for any reason.
Lastly, if at some point you want to reverse this modification, simply re-open the retractor and rotate the the orange gear counter-clockwise back to where it was and reassemble.