I went thought the same issues as you when trying to get a tow bar for my R33 4 door I spoke to about 10 tow bar places before I found one that would do it. I ended up getting it made up by Prestige tow bars in Melbourne for $300 phone (03) 9791 2566. They essentially copied Nissans design for the S14 200sx tow bar.
I actually purchased a S14 tow bar from Nissan for $200 thinking it would fit. Most of the Nissan RWD / AWD platforms are essentially the same in the chassis rail, spare wheel well, beaver panel area. The S14 tow bar mounts slide inside the chassis rail 22 cm with a piece on angle plating on the outside of the LH rail and two bolts going through and uses the tow point bolts on the other side. I believe the S14 kit will fit S13, S14, S13, R32 2 and 4 door, R33 2 door and maybe R34 and Stagea. It would not fit on my R33 4 door because they have a bodge in the beaver panel that stops the bar from being pushed all the way into the chassis rails. The S14 bar is ratted at 900kg.
The custom tow bar I had made up sits in the chassis rails the same way as the S14 kit but has a bend in the cross bar to clear the beaver panel bodge and just behind where the goose neck bolts on there is a bracing plate on the spare wheel well with two bolts going through to a plate on the inside on the wheel well. They rated it to 850kg. I think this rating is quite conservative as it looks as substantial if not more so than the BA falcon 1600kg tow bars that mount in the same manner.
I see no reason why this design could not be strengthened by extending the bars up a lot further inside the dog leg shaped rails to the rear sub frame mounting points. Bolted to shaped angle on the outside sandwedging the rails. You could even put a brace between the two rails just behind the rear subframe mounts with a third bar running up from behind the gooseneck mounting point along the boot floor under the fuel tank to meet this brace with bolts going through to plates on the inside.
The tow bar on my old VL that was rated to 1600kg was essentially just two plates one on ether side on the boot floor running up to just behind the back seats bolted together with a gooseneck mount hanging off the lower plate.
The pic shows the S14 tow bar