bnicho, that Draw-Tite one utilises factory mounting holes which have a nut attached to the backside for the top holes.
The lower holes (pictured with grey nut bracket thingies) require removal of two 10mm rear bumper bolts, drilling out the hole to 12mm and then bolting through (into the nut brackets). The metal bumper support bar also needs to be trimmed in the centre. This cannot be seen from behind the car though. 

 

 

 

 

Thanks for the pics knightrous.
 
Here is the installation guide for the Watling one. It appears to be braced to the engine mount also.

This is a rough cad of what I've been dreaming up.


 
At the T-bar, the plating is 15mm thick, has the holes for the safety chains and is where the tow ball tongue (Aim to use pre-engineered units) will bolt to. I would look at drilling 4 holes for the tongue to allow 2 different tow ball heights to give some flexibility in the varying trailers one might tow

Sweet! Would your design work for both N/A & SC bodied cars? Early N/A as well?
 
Are you seriously thinking of having your design built?

Just heard back from Watling in the UK. Towbar, 50mm ball & shipping to Australia is £244.75. That's about $415 AUD for a complete bolt on solution.

That's a decent price for a brand new towbar. Doesn't look like it has the crossbar though?
 
Knightrous; My GT's towbar bolted onto the chassis rails using 2x M12 bolts at either side, and that's rated to 800kg unbraked/1400kg braked, only thing is there is an outer bracket and the bolts are also clamping that to the rail as well.
As long as it's a high tensile bolt, it shouldn't matter too much for your small trailer?

is still a possibility?
 
I have always had the plan to get a tow bar setup eventually.
 
I did read ages ago that it was an OEM option in Canada, and that it was the same fitting as the Cressida (though I can't find that right now - forget if it was the MX73 or MX83 (for which you can get ADR approved aftermarket ones here:  http://www.towbarsaustralia.com.au/shop?type_1=towbar_kit&field_make_tid_selective=28&field_model_tid_selective_1=633 )

I'm still keen for a towbar but have done nothing about it to be honest. I'd happily buy an off the sheft Cressida one if I knew it would fit 100%

Potentially going to sell mine shortly on the preface that I have now bought a 4X4 to tow my car to the track instead.
Will post up here if I go through with it :P