The mystery of the Lost Coolant

Exactly. Thats what is was trying to say, just not very clearly. The ECU tells the IAC what RPM to idle at.
Which is why I don't believe the coolant lines do any more than prevent it freezing in cold weather. The IAC is just a servo operated spool valve to adjust airflow at idle based on what the ECU asks it to do. Don't think the IAC has any input itself into what speed to run at. 
From looking at the engine diagnostics when I was looking through the manual it looks like it's just the A/C compressor that calls for a high idle on the jimny. (700rpm A/C off vs 900rpm A/C on) 
I'll run it like it is for now and report back how it goes.

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Why would the throttle body freeze?  It's not like a carburettor, where you have evaporating fuel cooling the moisture in the air and blocking the venturi with ice.

That's why I'm not sure why it needs to be heated. The IAC does have a spool valve that could freeze if it was cold enough, cold damp air passing through the IAC will cause a pressure differential that could cause the valve to freeze. Welding CO2 regulators / flow meters are heated for the same reason. They will freeze up even on a warmish day due to the expansion of gas after passing through a small orifice. Under bonnet temps would increase to above freezing at roughly the same rate as engine coolant so could be a moot point. 
Its just a theory. I could be completely wrong about needing heating: I was simply asking if anyone else had done it, and if not, was going to see what happens if you do. 

Well, I didn't listen to my own advice and paid for it in time and frustration. Upside is I could probably set a world record removing an intake manifold from an M13A engine using nothing but ratchet ring spanners and a pair of pliers. 

Basically, I googled up the closest parts store open on a saturday that stocked the thermostat I needed. The one in there (that I didn't think was opening quite right, as per previous post) turns out wasn't the right one anyway (PO had replaced) and the gasket / o-ring doohickey appeared to be in bad shape. 
Anyway, where I left off last time I posted was reassembling everything with new parts and seeing what happened. I proceeded to do so and what I saw really slacked me off. Filled with water, cranked it up and watched as all the water disappeared onto the ground. Photo below shows what was happening once I pressure tested it with intake removed ( again ).
So out came the new t-stat and gasket, and I spent 2 - 3 hours trying every trick in the book to try and get it to seal. I even took the housing and cap to work and flattened the faces on the surface plate.
When I went to Suzuki to pick up my lower rad hose (they were the only place i could get one) I asked about thermostats and gaskets. They had them on the shelf so I grabbed one. Immediately on taking it out of the box when I got home, it was perfectly obvious what the issue was. The stupid aftermarket parts store gasket was never going to work. It was a different shape and way too thin to get any crimp in the groove in the housing. 
To add insult to injury, a complete thermostat and gasket from Suzuki was cheaper than the parts store one. After me preaching to everyone about using genuine parts, ironically its me that gets bitten again.
So after taking way longer than it needed to, and paying twice for a basic part I should have have just got from suzuki in the first place, its finally back together and so far is running just fine. I've been running plain water in it while testing, so once I have confirmed my problem is fixed i will fill with coolant and be back to surfing the dunes on another fishing trip. 

Oh, and also: bypassing the coolant (heater?) lines from the throttle body has so far had no detrimental affects whatsoever. Idles just fine as it always has and drops to 700 rpm after a minute or so of running like usual. So in my head I've gained at least 5hp from having cooler air in the manifold, but most likely the difference is pretty much zilch.

Cheers for the input guys, hopefully I have the problem beat and I won't be changing a head gasket anytime soon. 
   
 

Bummer. 

Yep, head gasket is toast. I used an extra strong radiator / cooling flush product to clean out the cooling system before filling with new coolant and heat cycled it quite a few times with the product in it and I think this is what finally made it give way properly. Foaming and bubbling when hot now out the rad neck. 
Oh well. I guess its due for timing chain anyway so I'll do that while I'm in there. And the valave cover gasket leaks so that will be fixed as well. Clutch will be easier to upgrade with engine out too. (Had this for over a year now and not got round to changing it yet).
​​​​Anyone done this and found any special tricks / pitfalls specific to these engines? I've had a number of engines apart (not these ones though - Diesel V8's) and back together again so the process itself is nothing new to me. I also have the service manual.

Only general advice - do the crank oil seal while it's accessible.  I would have thought the engine mounts would be tired too, if you are taking the whole thing out.